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CONNECTICUT 


•42 


AGRICULTURAL  EXPERIMENT 
STATION 


NEW  HAVEN,  CONN. 


BULLETIN  207  SEPTEMBER,  1918 


THE  EFFECTS  OF  INBREEDING 

AND  CROSSBREEDING  UPON 

DEVELOPMENT 


BY 

D.  F.  JONES 


The  Bulletins  of  this  Station  are  mailed  free  to  citizens  of  Connecti- 
cut who  applyTor  them,  and  to  others  as  far  as  the  editions  permit. 


CONNECTICUT  AGRICULTURAL  EXPERIMENT  STATION 

OFFICERS  AND  STAFF 


BOARD  OF  CONTROL. 
His  Excellency,  Marcus  H.  Holcomb,  ex-officio,  President. 

James  H.  Webb,  Vice  President Hamden 

George  A.  Hopson,  Secretary Wallingford 

E.  H.  Jenkins,  Director  and  Treasurer New  Haven 

Joseph  W.  Alsop Avon 

Wilson  H.  Lee Orange 

Elijah  Rogers Southington 

William  H.  Hall South  Willington 


Administration. 


Chemistry, 

Analytical  Laboratory, 


Protein  Research. 


Botany. 


Entomology. 


Forestry. 


Plant  Breeding. 


Vegetable  Growing. 


E.  H.  Jenkins,  Ph.D.,  Director  and  Treasurer. 

Miss  V.  E.  Cole,  Librarian  and  Stenographer. 

Miss  L.  M.  Brautlecht,  Bookkeeper  and  Stenographer. 

William  Veitch,  In  charge  of  Buildings  and  Grounds. 

*John  Phillips  Stree*t,  M.S. 

E.  Monroe  Bailey,  Ph.D.,  Chemist  in  charge. 

*C.  B.  Morison,  B.S.,  C.  E.  Shepard,   ] 

M.  d'Esopo,  Ph.B.  I    Assistants. 

H.  D.  Edmond,  B.S.  j 

Miss  A.  H.  Moss,  Clerk. 

V.  L.  Churchill,  Sampling  Agent. 

T.  B.  Osborne,  Ph.D.,  D.Sc,  Chemist  in  Charge. 
Miss  E.  L.  Ferry,  M.S.,  Assistant. 

G.  P.  Clinton,  Sc.D.,  Botanist. 

E.  M.  Stoddard,  B.S.,  Assistant  Botanist. 

Florence  A.  McCormick,  Ph.D.,  Scientific  Assistant. 

G.  E.  Graham,  General  Assistant. 

W.  E.  Britton,  Ph.D.,  Entomologist;   State  Entomologist. 

B.  H.  Walden,  B.Agr.,  First  Assistant. 

*I.  W.  Davis,  B.Sc,  M.  P.  Zappe,  B.S.,  Assistants. 
Miss  Martha  de  Bdssy,  B.A.,  Stenographer. 

Walter  O.  Filley,  Forester-   also  State  Forester 

and  State  Forest  Fire  Warden. 
A.  E.  Moss,  M.F.,  Assistant  State  and  Station  Forester. 
Miss  E.  L.  Avery,  Stenographer. 

Donald  F.  Jones,  S.D.,  Plant  Breeder. 

C.  D.  Hubbell,  Assista7it. 

W.  C.  Pelton,  B.S. 


*  Absent  on  leave.     In  service  of  the  United  States. 


CONTENTS 


Page 

Introduction 5 

Definitions .* 8 

Early  investigations  with  plants 9 

The  observations  of  Darwin  upon  plants 12 

Recent  investigations  with  plants 14 

Investigations  with  animals 18 

Universality  of  heterosis 21 

A  theoretical  consideration  of  inbreeding 22 

The  results  of  inbreeding  the  naturally  cross-pollinated  maize  plant  27 

The  approach  to  complete  homozygosity • .  .  .  .  44 

The  effect  of  heterozygosis  on  vegetative  luxuriance 47 

The  value  of  inbreeding  in  plant  and  animal  improvement 59 

The    effect    of    heterozygosis    upon    endosperm    development    and 

selective  fertilization 61 

The  effect  of  heterozygosis  upon  longevity,  hardiness  and  viability.  69 

The  effect  of  heterozygosis  upon  the  time  of  flowering  and  maturing.  76 

The  relation  of  the  effects  of  heterozygosis  and  of  the  environment.  78 

Summary  of  the  effects  of  inbreeding  and  crossbreeding 81 

A  Mendelian  interpretation  of  heterosis 82 

The  part  that  heterosis  has  played  in  the  establishment  of  sex 93 

Literature  cited 96 


The  Effects  of  Inbreeding  and  Crossbreed- 
ing Upon  Development* 


INTRODUCTION. 


Among  the  higher  seed  plants  certain  groups  are  characterized 
by  almost  universal  and  continuous  self-fertilization.  On  the 
other  hand  certain  other  groups  are  completely,  or  to  a  large 
extent,  cross-fertilized  in  every  generation.  Between  these  two 
extremes  every  gradation  in  the  degree  of  self-  and  cross-fertiliza- 
tion can  be  illustrated.  The  structure  and  function  of  the  floral 
organs  have  become  more  or  less  clearly  adapted  to  the  customary 
mode  of  sexual  reproduction  characteristic  of  each  species.  In 
the  thallophytes,  bryophytes  and  pteridophytes  much  the  same 
situation  exists  whereby  the  gametes  which  enter  into  a  sexual 
fusion  may  arise  either  from  the  same  or  from  different  organisms. 

In  the  lower  animals  the  same  variation  in  the  mode  of  sexual 
reproduction  exists  as  in  plants.  Among  the  higher  animals, 
however,  hermaphroditism  is  replaced  entirely  by  bisexuality;  and 
sexual  reproduction,  except  when  parthenogenesis  takes  place, 
results  only  from  the  union  of  gametes  originating  in  different 
organisms. 

This  array  of  facts  has  naturally  led  to  searching  inquiries  as 
to  the  purpose  of  sexual  reproduction  as  compared  to  other 
methods  of  propagation  as  well  as  to  the  effects  of.  artificial  in- 
breeding in  bisexual  animals  and  in  naturally  cross-fertilized 
plants.  Bound  up  with  this  latter  problem  is  that  which  is  con- 
cerned with  the  effects  of  cross-fertilization  in  all  types  of  animals 
and  plants  of  different  degrees  of  relationship. 

The  development  of  the  Mendelian  theory  of  heredity,  carrying 
with  it  the  conception  of  definable,  hereditary  units  which  are 
sufficiently  stable  in  their  transmission  from  generation  to  genera- 
tion to  be  recognized  and  their  somatic  expression  to  be  described, 


*  Submitted  to  the  Faculty  of  the  Bussey  Institution  of  Harvard 
University  in  partial  fulfillment  of  the  requirements  for  the  degree  of 
Doctor  of  Science,  December,  1917. 


6  CONNECTICUT    EXPERIMENT   STATION    BULLETIN    207. 

lias  made  possible  an  attack  upon  these  problems  which  has 
opened  a  way  towards  their  solution. 

From  the  knowledge  of  alternate  inheritance  it  is  possible  to 
ascribe,  very  definitely  and  surely,  certain  of  the  results  of  in- 
breeding to  the  segregation  and  isolation  of  hereditary  factors 
which  results  were  formerly  thought  to  be  due  solely  to  inbreeding 
as  a  cause  in  itself.  Certain  pathological,  abnormal  or  otherwise 
undesirable  conditions  occurring  more  frequently  in  animals 
and  plants  produced  by  matings  between  nearly  related  individuals 
were  formerly  attributed  to  inbreeding  as  the  cause,  and  it  was 
thought  that  inbreeding  must  always  show  such  undesirable 
results.  It  is  now  known  that  many  of  these  pathological  and 
abnormal  conditions  resulting  from  inbreeding  do  not  owe  their 
origin  to  that  process,  but  are  due  solely  to  the  segregation,  into 
a  pure  state  of  the  hereditary  factors  causing  the  anomalies 
which  factors  were  present  in  the  organisms  previous  to  their 
being  inbred.  Inbreeding,  then,  has  nothing  to  do  with  the 
origin  of  the  undesirable  characters  under  consideration  but 
merely  brings  them  into  visible  expression,  and  whether  or  not 
they  appear  depends  upon  their  presence  originally  in  the  stock 
before  inbreeding  takes  place.  There  still  remains  a  conviction, 
however,  that  all  the  manifestations  attending  inbreeding  and 
the  converse  effects  of  cross  breeding  cannot  be  accounted  for 
solely  on  the  basis  of  the  operation  of  definable,  hereditary  factors, 
but  that  there  is  a  stimulating  effect  resulting  from  crossing, 
which  is  lost  by  inbreeding,  and  that  this  stimulation  differs 
somewhat  from  the  expression  of  hereditary  factors  which  can 
be  transferred  and  fixed  in  different  organisms.  This  stimulation 
is  supposed  to  be  of  a  physiological  nature  appearing  when  dis- 
similar germ-plasms  are  united,  and  disappearing  as  the  germinal 
heterogeneity  disappears  in  subsequent  recombinations. 

Since  this  physiological  stimulation  has  always  been  purely 
hypothetical,  having  never  been  definitely  proven,  and  since  it 
has  been  used  to  account  for  certain  facts  heretofore  inexplicable 
in  any  other  way,  the  existence  of  such  a  stimulation  may  fairly 
be  questioned,  in  so  far  as  the  facts  can  be  logically  accounted 
for  in  other  ways.  Recent  advances  in  the  knowledge  of  the 
methods  of  inheritance  have  made  it  possible  to  meet  certain 
objections  previously  held  against  the  view  that  the  effects  of 
inbreeding   and     crossbreeding   can  be  attributed  solely  to  the 


INTRODUCTION.  / 

operation  of  hereditary  factors  without  assuming  an  additional 
hypothetical  stimulation. 

Some  of  the  previous  work  bearing  upon  the  effects  of  inbreed- 
ing and  crossbreeding  is  reviewed  here  and  with  this  are  given 
original  data  obtained  from  the  naturally  cross-fertilized  corn 
plant,  Zea  mays  L.  The  facts  at  hand  co-ordinate  with  the  exist- 
ing knowledge  of  heredity  in  such  a  way  that  it  seems  to  the 
writer  unnecessary  any  longer  to  make  the  fundamental  dis- 
tinction between  the  effects  of  inbreeding  and  crossbreeding  and 
of  heredity  in  development.  , , 

No  attempt  is  made  to  canvas  the  extensive  literature  on 
hybridization  (a  bibliography  of  which  alone  would  fill  a  volume) 
in  order  to  list  all  the  cases  in  which  crossing  does  or  does  not 
result  in  increased  development  and  inbreeding  in  a  reduction. 
It  does  not  take  one  long  in  reading  over  the  many  published 
results  of  crossing  in  animals  and  plants  to  become  convinced 
that  an  increase  in  development  following  a  cross  is  a  frequent 
occurrence.  It  is  hoped  that  sufficient  references  are  given  to 
show  something  as  to  the  universality  and  nature  of  the  phenom- 
enon and  a  review  of  the  more  important  contributions  is  made 
in  order  to  sketch  briefly  the  development  of  the  ideas  concerning 
the  cause  of  the  stimulation  and  the  part  it  has  played  in  evo- 
lution and  in  breeding  practice. 

The  experiments  on  inbreeding,  which  have  resulted  in  the 
material  from  which  the  data  given  here  have  been  gathered, 
were  started  by  Professor  E.  M.  East  at  the  Connecticut  Agri- 
cultural Experiment  Station  and  carried  on  by  him  and  subse- 
quently by  Professor  H.  K.  Hayes  and  later  by  the  writer.  From 
time  to  time  reports  on  these  experiments  have  been  made  and 
conclusions  drawn  from  the  facts  as  observed.  These  include 
various  publications  under  the  titles  "  Inbreeding  in  Corn," 
"  The  Distinction  between  Development  and  Heredity  in  In- 
breeding "  published  by  Professor  East  in  the  Report  of  the 
Connecticut  Experiment'  Station  and  in  the  American  Naturalist 
and  "  Heterozygosis  in  Evolution  and  in  Plant  Breeding  "  by 
Professors  East  and  Hayes  in  a  Bureau  of  Plant  Industry  bulletin. 
Under  the  title  of  "  Dominance  of  Linked  Factors  as  a  Means 
of  Accounting  for  Heterosis  "  the  writer  had  proposed  a  different 
view  as  to  the  cause  of  hybrid  vigor.  This  was  published  in 
Genetics  and  its  application  is  discussed  here  in  more  detail. 


8  CONNECTICUT   EXPEEIMENT   STATION   BULLETIN   207. 

Further  publications  are  planned  which  will  discuss  more  ade- 
quately much  of  the  data  which  are  scantily  treated  here. 

The  significance  which  these  investigations  may  have  for  the 
practical  improvement  of  plants  and  animals  has  only  been  briefly 
alluded  to  here.  This  phase  of  the  subject  has  been  reserved 
for  another  time  when  the  methods  which  have  suggested  them- 
selves as  the  result  of  these  investigations  have  been  more  thor- 
oughly tested.  Finally  this  collection  of  facts  and  theories  should 
be  viewed  as  a  report  of  progress  rather  than  a  well  rounded 
presentation  of  the  subject  of  inbreeding  and  crossbreeding. 

The  writer  is  especially  indebted  to  his  predecessors  whose 
work  has  made  these  experiments  possible.  Grateful  acknowl- 
edgement is  due  Dr.  E.  M.  East  for  his  careful  supervision  of  the 
work  and  for  his  kindly  advice  and  helpful  criticism  as  to  the 
presentation  of  the  results  obtained.  The  writer  alone,  however, 
must  assume  the  responsibility  for  the  opinions  expressed.  Much 
credit  is  due  Mr.  C.  D.  Hubbell,  Dr.  Charles  Drechsler  and  Mr. 
G.  A,  Adsit  for  their  careful  assistance  in  the  collection  and 
•  preparation  of  the  data. 

Definitions. 

The  knowledge  of  a  stimulating  effect  resulting  from  a  cross 
between  different  animals  and  between  different  plants  which 
gives  progeny  which  may  excel  their  parents  in  general  vigor, 
size  or  other  visible  characteristics  has  naturally  led  to  the  use 
of  terms  to  describe  this  effect.  This  stimulation  is  variously 
spoken  of  as  "vigor  due  to  crossing"  or  "hybrid  vigor."  Since 
hybrid  vigor  occurs  only  in  crosses  of  which  the  parents  are  dis- 
similar in  hereditary  constitution  more  exact  and  comprehensive 
terms  were  needed.  The  zygote  resulting  from  a  union  of  unlike 
gametes  is  spoken  of  as  a  heterozygote  (following  the  usage  of 
Bateson),  hence  the  term  heterozygosis  (used  by  Spillman,  '09) 
refers  to  that  germinal  heterogeneity  which  results  from  the  union 
of  unlike  gametes,  and  the  stimulation  to  development  which  accom- 
panies such  a  condition  is  spoken  of  as  a  "stimulus  of  heterozy- 
gosis," or  "heterozygotic  stimulation,"  meaning  the  stimulating 
effects  of  hybridity  or  the  stimulation  due  to  differences  in  uniting 
gametes.  The  converse  fact  of  a  reduction  in  vigor  accompanying 
a  return  to  a  homozygous  condition  is  therefore  said  to  be  due  to, 


EARLY   INVESTIGATIONS   WITH    PLANTS.  9 

or  result  from,  homozygosis.  Shull  ('14)  has  proposed  the  term 
"heterosis"  to  designate  this  increase  in  development  which  may 
result  from  a  heterozygous  condition;  hence,  heterosis,  as  used 
here,  will  be  considered  synonymous  with  "hybrid  vigor"  or 
"stimulus  accompanying  heterozygosis,"  in  whatever  form  this 
may  be  manifested  or  whatever  cause  or  causes  it  may  be  due  to. 
Shull  proposed  this  term,  as  he  says,  "...  .to  avoid  the  implication 
that  all  the  genotypic  differences  which  stimulate  cell-division, 
growth,  and  other  physiological  activities  of  an  organism,  are 
Mendelian  in  their  inheritance  and  also  to  gain  in  brevity  of  ex- 
pression. ..."  Hence  the  term  heterosis  is  not  meant  as  a  mere 
contraction  of  heterozygosis  and  is  not  synonymous  with  it.  The 
adjective  "heterotic"  has  also  been  proposed  and  such  an  ex- 
pression as  "heterotic  stimulation"  is  synonymous  with  heterosis. 

Early  Investigations  with  Plants. 
Certain  evidence'  remains  from  the  carvings  of  the  ancient 
Egyptians  to  show  that  they  had  some  conception  of  a  sexuality 
in  plants.  However,  it  was  not  until  the  last  of  the  17th  century, 
when  Camerarius  first  demonstrated  such  condition,  that  interest 
in  the  production  of  artificial  hybrids  began.  Tt  is  significant 
that  the  first  artificial  hybrids  to  be  systematically  studied, 
those  of  Kolreuter  (1776),  furnished  some  of  the  best  examples  of 
heterosis.  Kolreuter  made  many  interspecific  crosses  in  Nicotiana, 
Dianthus,  Verbascum,  Mirabilis,  Datura  and  others,  many  of 
which  astonished  their  producer  by  their  greater  size,  increased 
number  of  flowers  and  general  vegetative  vigor,  as  compared  to" 
the  parental  species  entering  into  the  cross.  Concerning  one  of 
the  tobacco  crosses  he  says:  (pp.  57-58)  "Hybrids  obtained 
from  the  cross  of  Nicotiana  maj.  9  and  glut,  d71  produced  a  far 
greater  number  of  flowers  and  grew  to  an  uncommonly  greater 
height  and  a  much  greater  circumference  than  the  pure  species 
under  the  same  conditions ;  the  height  of  the  plants  which  were 
kept  in  the  hot  bed  or  were  set  out  in  the  field  after  they  had  ob- 
tained full  growth,  amounted  to  eight  feet  and  1  to  10  inches; 
the  whole  circumference  of  the  branches  to  24  feet;  the  largest 
diameter  of  the  stalks  from  2  inches  to  2  inches  and  3  lines;  and 
the  largest  leaves  were  2  feet,  2  inches  and  9  lines  long  and  1  foot 
and  4  inches  wide.  Never  has  anyone  seen  more  magnificent 
tobacco  plants  than  these  were." 


10  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

Thomas  Andrew  Knight  (1799)  was  one  among  several  at  that 
time  who  experimented  with  hybrids  with  the  view  of  producing 
more  desirable  varieties  of  vegetables,  flowers  and  fruits.  Knight 
observed  many  instances  of  high  vigor  resulting  from  crossing; 
among  these  we  note  the  following  remarks  about  a  cross  between 
two  varieties  of  peas. 

(P.  200)  "By  introducing  the  farina  of  the  largest  and  most  luxuriant 
kinds  into  the  blossoms  of  the  most  diminutive  and  by  reversing  this 
process,  I  found  that  the  powers  of  the  male  and  female  in  their  effects 
on  the  offspring,  are  exactly  equal.  The  vigor  of  the  growth,  the  size  of 
the  seeds  produced,  and  the  season  of  maturity,  were  the  same,  though 
the  one  was  a  very  early,  and  the  other  a  very  late  variety.  I  had,  in 
this  experiment,  a  striking  instance  of  the  stimulative  effects  of  crossing 
the  breeds;  for  the  smallest  variety,  whose  height  rarely  exceeded  two 
feet,  was  increased  to  six  feet;  whilst  the  height  of  the  large  and  luxuriant 
kind  was  very  little  diminished." 

It  is  evident  that,  in  these  crosses,  Knight  was  dealing  with 
dwarf  and  standard  peas  and  the  dominance  of  standardness  is 
expected.  A  sufficient  number  of  cases,  however,  were  observed 
in  which  the  crosses  were  more  vigorous  than  an  average  of  the 
parents  to  convince  him  that  "nature  intended  that  a  sexual 
intercourse  should  take  place  between  neighboring  plants  of  the 
same  species."  It  was  this  principle  which  Darwin  elaborated 
50  years  later. 

Sageret  ('26)  reports  vigorous  hybrids  in  Nicotiana  and  also 
between  different  types  of  the  Cucurbitaceae.  Among  other 
'things  he  notes  that  in  human  crosses  between  one  individual 
which  shows  a  hereditary  pathological  condition  and  a  normal 
individual,  that  the  disease  disappeared  in  the  first  generation 
but  reappeared  in  the  second  and  following  generations.  Wiegmann 
('28)  gives  instances  of  hybrids  in  the  Cruciferae  which  showed 
distinct  evidences  of  heterosis. 

Probably  the  most  extensive  series  of  experiments  on  hybridiza- 
tion were  those  of  Gartner  ('49)  and  of  Focke  ('81).  According 
to  Lindley  ('52)  Gartner  made  10,000  crosses  between  700  different 
species  and  produced  250  different  hybrids.  Many  of  these  hy- 
brids showed  distinct  evidences  of  heterosis,  and  this  phenomenon 
was  manifested  in  many  different  ways.  Gartner  speaks  especially 
of  their  general  vegetative  luxuriance,  increase  in  root  develop- 
ment, in  height,  in  number  of  flowers  and  their  hardiness  and  early 


EAELY   INVESTIGATIONS   WITH    PLANTS.  11 

and  prolonged  blooming.  Focke  made  equally  extensive  observa- 
tions and  catalogues  his  own  experiments  with  many  of  those 
made  previously.  His  valuable  book  shows  clearly  that  the  phe- 
nomenon of  heterosis  is  widespread  and  may  be  expected  in  the 
gymnosperms  and  pteridophytes  as  well  as  in  the  angiosperms. 
Both  the  works  of  Gartner  and  of  Focke  have  been  so  thoroughly 
reviewed  in  recent  times  (East  and  Hayes  '12)  in  connection  with 
the  problem  in  hand  that  it  would  be  a  needless  repetition  to  say 
more  about  their  results  here.  Special  points  in  their  observations, 
as  they  supplement  the  experiments  recorded  here,  will  be  referred 
to  later. 

While  the  work  of  Gartner  and  Focke  must  always  rank  high  as 
contributions  to  our  knowledge  of  genetics  one  cannot  refrain 
from  remarking  that  they  both  missed  by  their  extensive  studies 
of  many  species  the  point  which  Mendel  discovered  by  his  inten- 
sive and  careful  study  in  one  species. 

Naudin  ('65)  next  to  Mendel  will  always  be  remembered,  no 
doubt,  as  the  first  to  conceive  of  a  method  in  the  uniformity  of 
the  first  generation  and  the  variability  of  the  second.  His  con- 
ception of  the  segregation  of  parental  qualities  as  a  whole  leads 
up  naturally  to  Mendel's  law  whereby  the  characters  of  the 
parents  segregate  as  units  and  when  finally  appreciated  the 
chaotic  observations  of  Gartner,  Focke  and  their  contemporaries 
began  to  be  understood  as  orderly  facts.  In  Naudin's  classical 
experiments  there  are  many  excellent  examples  of  heterosis. 
Out  of  36  interspecific  crosses  which  he  made  in  Papaier, 
Mirabilis,  Primula,  Datura,  Nicotiana,  Petunia,  Digitalis,  Linaria, 
Luffa,  Coccinea  and  Cucumis,  24  show  positive  evidence  of  het- 
erosis. Among  the  most  notable  crosses  in  this  respect  was  that 
of  Datura  Stramonium  with  D.  Tatula  in  which  both  reciprocal 
hybrids  were  twice  as  tall  as  either  parent.  Concerning  the 
Datura  crosses  Naudin  says : 

"A  shape  very  much  taller  than  the  two  parental  types,  and  the  pre- 
mature falling  off  of  the  flowers  in  the  first  dichotomies,  which  leads  to 
tardy  fructification  are  the  principal  characteristics  of  this  hybrid  of 
which  all  the  plants  in  the  collection  present  the  greatest  uniformity. 
We  shall  see  that  these  different  characteristics  appear  in  all  the  hybrids 
of  this  section  of  the  genus  Datura." 

Mendel  ('65)  also  records  instances  of  heterosis  in  his  pea 
hybrids  as  is  shown  in  the  following  passage : 


12  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

"The  longer  of  the  two  parental  stems  is  usually  exceeded  by  the 
hybrid,  a  fact  which  is  possibly  only  attributable  to  the  greater  luxuriance 
which  appears  in  all  parts  of  plants  when  stems  of  very  different  lengths 
are  crossed.  Thus,  for  instance,  in  repeated  experiments,  stems  of  1  foot 
and  6  feet  in  length  yileded  without  exception  hybrids  which  varied  in 
length  between  6  feet  and  7K  feet." 

The  Observations  of  Darwin  upon  Plants. 

Of  all  the  contributors  to  our  knowledge  of  the  effects  of  in- 
breeding and  crossbreeding  no  one  has  collected  as  many  facts 
as  Darwin  ('75,  '77).  Although  undoubtedly  much  confusion 
and  misunderstanding  have  resulted  from  Darwin's  conclusions 
on  this  problem,  one  cannot  but  admire  his  painstaking  efforts 
to  accumulate  facts  from  the  behavior  of  many  species  of  plants 
through  many  generations  of  crossing  and  selfing  before  advancing 
his  conclusions.  No  one  was  more  frank  to  acknowledge  the 
discrepancies  between  the  facts  as  he  found  them  and  the  con- 
clusions he  drew  from  them.  Those  parts  of  his  results  which 
were  not  clear  to  Darwin  are  clearer  to  us  through  our  knowledge 
of  Mendelism  of  which  he  was  not  permitted  to  know.  Since 
his  method  of  experimentation,  and  the  results  obtained  are 
familiar  to  all  interested  in  the  problem  at  hand  no  extensive 
review  of  his  work  is  necessary.  Only  a  brief  summary  of  the 
results  obtained  and  the  conclusions  which  he  drew  from  them 
will  be  given  here,  reserving  a  more  detailed  review  of  special 
parts  for  a  later  part  of  this  paper. 

Among  animal  breeders  in  Darwin's  time  it  was  a  common 
belief  that  whatever  evil  effects  resulted  from  more  or  less  close 
inbreeding  were  due  to  the  accumulation  of  abnormal,  diseased, 
or  morbid  tendencies  in  the  offspring  of  parents  which  possessed 
such  tendencies.  Darwin  refused  to  ascribe  any  large  part  of  the 
effects  of  inbreeding  to  this  cause  because  he  knew  so  many  cases 
were  weakened  and  reduced  types  of  both  plants  and  animals 
which  gave  vigorous  progeny  when  crossed  among  themselves. 
Instead  of  an  accumulation  of  the  undesirable  traits  of  both 
parents  the  very  reverse  seemed  to  be  true.  Had  Darwin  known 
of  the  way  by  which  recessive  characters  may  exist  for  many 
generations  without  making  their  appearance,  doubtless  his  views 
on  this  point  would  have  differed  materially. 

Darwin  clearly  thought  that  the  evil  effects  of  inbreeding  kept 
on  accumulating  until  eventually  a  plant  or  animal  propagated 


THE  OBSERVATIONS  OF  DARWIN  UPON  PLANTS.        13 

in  that  manner  was  doomed  to  extinction.  His  Own  results  came 
far  short  of  proving  such  an  assumption.  The  two  wild  plants 
with  which  inbreeding  was  practiced  the  longest — Ipomea  and 
Mimulus — showed  very  little  further  loss  of  vigor  after  the  first 
generation.  What  these  experiments  did  show,  most  clearly, 
was  that  there  was  segregation  of  the  inbred  stock  into  diverse 
types  which  differed  in  minor,  visible,  heriditary  characters  and 
which  also  differed  in  their  ability  to  grow.  In  both  species 
plants  appeared  which  were  superior  to  other  plants  derived 
from  the  same  source  and  some  were  even  equal  or  superior  in 
vigor  to  the  original  cross-pollinated  stock.  They  differed  from 
this  race,  however,  most  noticeably  in  the  uniformity  of  all 
visible  characteristics. 

After  several  generations  of  inbreeding  Darwin  found  that  it 
made  no  difference  in  the  resulting  vigor,  whether  the  plants  in 
an  inbred  lot  were  selfed  or  were  crossed  among  themselves. 
This  he  correctly  attributed  to  the  fact  that  the  members  of 
such  an  inbred  strain  had  become  germinally  alike.  From  his 
views  on  the  effect  of  the  environment  on  organisms,  it  is  easy 
to  see  why  he  attributed  this  approach  to  similarity  in  inherited 
qualities  to  the  fact  that  the  plants  were  grown  for  several 
generations  under  the  same- conditions.  This  view  he  thought 
was  supported  by  the  fact  that  crosses  of  his  selfed  lines  with 
the  intercrossed  lines  (also  inbred,  but  to  a  less  degree)  did  not 
give  as  great  increase  in  vigor  as  the  crosses  of  either  lines  with 
a  fresh  stock  from  distant  regions.  The  crosses  between  two 
inbred  lines  did  give  a  noticeable  increase  in  vigor,  in  many 
cases,  equaling  the  original  variety.  This  is  illustrated  in  the 
Dianthus  crosses  in  which  the  selfed  line  was  crossed  with  the 
intercrossed  line  and  with  a  fresh  stock.  The  ratio  of  both 
crosses  to  the  selfed  plants  in  height,  number  of  capsules  and 
weight  of  seed  produced  is  as  follows: 

Selfed  Selfed 

X  X 

Inter-crossed  Fresh  stock 

Height,  compared  to  selfed 100:95  100:81 

No.  Capsules,  compared  to  selfed 100:67  100:39 

Weight  of  seed,  compared  to  selfed 100:73  100:33 

Like  Darwin  we  now  attribute  the  greater  increase  of  vigor 
in  a  cross  with  distinct  stocks  to  a  greater  germinal  diversity 
although  we  may  differ  in  our  ideas  as  to  the  way  in  which  that 


14  CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 

diversity  was  brought  about.  Whatever  may  be  the  explanation 
of  that,  credit  is  due  Darwin  for  being  the  first  to  see  that  it  was 
not  the  mere  act  of  crossing  which  induced  vigor  but  the  union 
of  different  germinal  complexes.  This  he  states  clearly  in  the 
following  sentences  (Cross  and  Self  Fert.,  p.  270) : 

"  These  several  cases  taken  together  show  us  in  the  clearest  manner 
that  it  is  not  the  mere  crossing  of  any  two  individuals  which  is  beneficial 
to  the  offspring.  The  benefit  thus  derived  depends  on  the  plants  which 
are  united  differing  in  some  manner,  and  there  can  hardly  be  a  doubt 
that  it  is  in  the  constitution  or  nature  of  the  sexual  elements.  Anyhow, 
it  is  certain  that  the  differences  are  not  of  an  external'  nature,  for  two 
plants  which  resemble  each  other  as  closely  as  the  individuals  of  the  same 
species  ever  do,  profit  in  the  plainest  manner  when  intercrossed,  if  their 
progenitors  have  been  exposed  during  the  several  generations  to  different 
conditions." 

Recent  Investigations  with  Plants. 

Although  Darwin  was  the  first  to  attack  the  problem  from  the 
standpoint  of  determining  the  effects  of  inbreeding,  it  is  doubtful 
if  he  clearly  recognized  that  the  same  phenomenon  was  concerned 
in  both  inbreeding  and  crossbreeding.  It  remained  for  Shull  ('08, 
'09,  '10,  '11  and  '14),  East  ('08,  '09)  and  East  and  Hayes  ('12) 
to  bring  out  clearly  the  fundamental  similarity  of  both  processes 
and  to  put  the  matter  in  such  a  light  that  a  far  clearer  under- 
standing of  the  nature  of  the  effects  of  inbreeding  has  resulted. 

Their  conclusions  in  regard  to  the  causes  of  the  effects  of 
inbreeding  and  crossing  were  for  the  most  part  entirely  new  and 
dependent  for  their  support  upon  the  Mendelian  principle  of  the 
segregation  and  recombination  of  inherited  qualities  as  inde- 
pendent units  and  upon  Johannsen's  genotype  conception  of 
heredity.  Stated  briefly  their  main  tenets,  based  upon  their 
own  careful  experiments  and  a  survey  of  previous  results  bearing 
upon  the  problem,  are  as  follows: 

1.  Inbreeding  automatically  sorts  out  into  homozygous,  pure 
breeding  lines,  the  diverse  and  variating  complex  of  hereditary 
characters  found  in  a  naturally  cross-pollinated  species. 

2.  Although  complete  homozygosity  is  difficult  to  attain  in 
practice,  after  several  generations  of  selfing,  members  of  the 
resulting  inbred  lines  are  uniform  among  themselves  but  the 
respective  lines  may  differ  greatly  among  each  other  in  visible 


RECENT   INVESTIGATIONS   WITH    PLANTS.  15 

hereditary  characters.  The  strains  may  also  differ  in  their  power 
of  development,  some  being  larger,  stronger  and  more  productive 
than  others  at  normal  maturity.  Some  individuals  are  often 
isolated  which  are  so  lacking  in  necessary  characters  that  they 
perish  because  of  inability  to  reproduce  themselves. 

3.  Those  inbred  strains  which  are  able  to  survive  finally  be- 
come constant;  no  further  reduction  in  vigor  or  change  in  visible 
characters  is  to  be  expected  by  continued  inbreeding.  These 
constant  types  are  thus  quite  comparable  to  naturally  self- 
fertilized  species  and  may  exist  indefinitely. 

4.  When  these  pure  breeding  types  are  crossed  there  is  com- 
monly an  immediate  and  striking  increase  in  general  size  and 
vigor  to  be  expected  in  the  resulting  first  hybrid  generation. 

To  account  for  this  increase  in  development,  following  a  cross, 
a  physiological  stimulation  was  postulated  which  accompanied 
heterozygosity  of  hereditary  factors  and  disappeared  as  the 
organisms  approached  homozygosity.  As  an  illustration  the 
union  of  factor  "A"  with  it  allelomorph  "a"  was  considered  to 
evolve  developmental  'energy  which  was  lacking  when  either 
"A"  or  "a"  were  united  with  themselves.  This  stimulus  to  devel- 
opment was  considered  to  be  due  to  the  union  of  unlike  factors 
alone  and  to  have  an  effect  quite  different  from  whatever  part 
each  factor  had  by  itself  in  the  development  of  the  organism. 
Stated  in  their  own  words  the  main  conclusions  of  East  and 
Hayes  ('12)  are  as  follows  (p.  8): 

"  1.  Mendel's  law — "that  is,  the  segregation  of  character  factors  in 
the  germ  cells  of  hybrids  and  their  chance  recombinations  in  sexual 
fusions — is  a  general  law. 

2.  Stimulus  to  development  is  greater  when  certain,  or  possibly  all, 
characters  are  in  the  heterozygous  condition  than  when  they  are  in  a 
homozygous  condition. 

3.  This  stimulus  to  development  is  cumulative  up  to  a  limiting  point 
and  varies  directly  with  the  number  of  heterozygous  factors  in  the 
organism  although  it  is  recognized  that  some  of  the  factors  may  have  a 
more  powerful  action  than  others." 

It  was  clearly  apparent  to  recent  investigators  that  many  of 
the  unfavorable  characters  which  appear  on  inbreeding  a  naturally 
cross-pollinated  species  are  recessive  characters  which  are  segre- 
gated out  of  the  original  complex.  In  a  naturally  crossed  species, 
these  are  hidden  from  sight  on  account  of  being  continually 


16  CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 

crossed  with  dominant  characters.  That  dominance  of  factors 
could  in  any  way  be  an  essential  factor  in  the  vigor  and  excellence 
of  hybrids,  an  idea  first  proposed  by  Keeble  and  Pellew  ('10) 
and  also  by  Bruce  ('10),  has  not  been  accepted  by  most  writers 
on  this  subject.  They  considered  dominance  to  be  totally  in- 
adequate to  account  for  the  widespread  and  almost  universal 
occurrence  of  heterosis  in  plants  and  animals  and  the  fact  that 
nearly  all  naturally  cross-fertilized  domesticated  species  are 
reduced  by  inbreeding. 

Collins  ('10)  has  shown  clearly  that  many  crosses  between 
varieties  of  Indian  corn  already  widely  crossed  among  themselves 
and  grown  in  the  same  regions  may  not  give  any  increase  in 
productiveness,  but  when  these  same  varieties  are  crossed  with 
varieties  from  distinct  geographical  regions  great  increases  in 
productiveness  are  obtained.  Further  evidence  as  to  the  occurrence 
of  heterosis  is  seen  in  the  many  publications  which  have  appeared 
from  time  to  time  urging  the  commercial  utilization  of  this  hybrid 
vigor  as  a  method  of  increasing  production  in  many  plants. 
Among  these  are  Beal  (76-  82),  McCleur  ('92),  Morrow  and 
Gardner  ('93-94),  Swingle  and  Webber  ('97),  Hayes  and  East 
('11),  Hartley  ('12),  Wellington  ('12),  Hayes  ('13),  Hayes  and 
Jones  ('16). 

In  view  of  the  innumerable  cases  in  which  an  increase  in  devel- 
opment, in  some  character,  results  from  crossing  and  the  converse 
fact  of  reduction  following  subsequent  inbreeding,  of  which  the 
preceding  paragraphs  refer  to  only  a  small  fraction,  it  is  surpris- 
ing to  note  such  radically  diverse  opinions  as  are  held  by  Burck 
{'08)  and  championed  by  Stout  ('16). 

Stout  attributes  the  following  statements  to  Burck:    (p.  418) 

"That  (1)  plants  that  are  regularly  self-fertilized  show  no  benefits 
from  crossing  and  that  (2)  nowhere  in  wild  species  is  there  evidence  of 
an  injurious  effect  from  self-fertilization,  and  that  there  is  abundant 
evidence  of  continued  vigor  and  high  fertility  resulting  from  long  con- 
tinued self-fertilization." 

If  by  the  first  statement  is  meant  that  crossing  between  members 
of  the  same  variety  or  between  individuals  of  a  uniform  species 
does  not  give  an  increase  in  development  such  a  result  would  be 
expected  because  of  the  germinal  similarity  brought  about  by 
long  continued  selfing  and  elimination  by  selection,  either  natural 


EECENT    INVESTIGATIONS   WITH   PLANTS.  17 

or  artificial,  of  all  but  one  type.  On  the  other  hand,  there  is 
abundant  evidence  to  show  that  crossing  between  different  vari- 
eties or  between  different  wild  species  of  self-pollinated  plants 
often  results  in  striking  increases  in  size  and  vigor.  It  is  only 
necessary  to  refer  to  the  work  of  Kolreuter,  Knight,  Gartner, 
Naudin  and  Mendel  where  many  crosses  between  different  species 
or  between  distinct  types  of  Nicotiana,  Pisum,  and  Lathyrus — 
plants  which  are  naturally  self-fertilized — give  unmistakable 
evidence  of  heterosis. 

Turning  to  the  effects  of  inbreeding,  almost  no  long-continued 
experiments  have  been  carried  out  with  strictly  wild  cross-polli- 
nated species  of  plants.  Collins  ('18)  in  a  brief  note  states  that 
teosinte,  a  semi-wild  relative  of  maize,  is  not  affected  by  in- 
breeding to  the  extent  that  maize  is.  That  there  is  "abundant 
evidence  of  continued  vigor  and  high  fertility  resulting  from 
long  continued  self-fertilization"  no  one  longer  doubts.  There  is, 
however,  hardly  enough  evidence  from  plants,  so  far  on  record, 
to  justify  the  sweeping  statement,  which  the  quotation  implies, 
that  cross-fertilized  wild  species  are  never  reduced  by  inbreeding. 

What  evidence  there  is  indicates  that  naturally  crossed  wild 
species  are  not  reduced  by  inbreeding  to  anything  like  the  extent 
that  domesticated  races  are.  More  will  be  said  about  this  differ- 
ence between  wild  and  domesticated  races  later.  There  is  some 
evidence,  however,  to  show  that  strictly  wild  species  are  affected 
by  inbreeding.  Darwin  compared  the  progeny  of  artificially  self- 
fertilized  plants  with  the  progeny  of  artificially  intercrossed 
plants  of  many  wild  species.  Many  of  these  species  were  such  as 
were  for  one  cause  or  another  almost  completely  cross-fertilized 
in  their  natural  state  at  all  times.  Although  the  difference  may 
be  slightly  exaggerated  there  can  be  no  question  but  that  the 
difference  in  the  first  generation  which  Darwin  obtained  between 
the  selfed  plants  and  the  intercrossed  plants  represents  in  many 
cases  the  effect  which  inbreeding  has  upon  these  plants.  As 
examples  of  widely  crossed  wild  species  in  which  a  reduction  in 
the  first  generation  of  inbreeding  was  obtained  by  Darwin,  one 
can,  therefore,  cite:  Digitalis  purpurea,  Linaria  vulgaris,  Saro- 
thamnus  scoparius  and  Reseda  lutea. 

Moreover,  no  matter  how  much  domestication  may  change 
plants  from  the  wild,  one  cannot  cast  aside,  as  of  no  consequence, 
the  results  obtained  from  cultivated  plants. 


18         connecticut  experiment  station  bulletin  207. 

Investigations  with  Animals. 

According  to  Darwin,  the  mule,  that  classic  example  of  hybrid 
vigor,  was  known  in  the  time  of  Moses,  when  its  hardihood  and 
general  good  qualities  doubtless  endeared  this  animal  to  the  Jews 
no  less  than  to  the  Southern  cotton  planters  of  to-day.  A  similar 
cross  of  the  ass  with  the  wild  zebra  according  to  Riley  ('10)  gives 
a  first  generation  hybrid  animal  of  considerable  merit. 

In  the  early  history  of  the  establishment  and  fixation  of  breeds 
of  livestock  we  note  in  Darwin's  "Animals  and  Plants  under 
Domestication"  that  certain  crosses  between  different  breeds  often 
resulted  in  progeny  excelling  individuals  of  either  parent  breed; 
just  as  to-day  it  is  not  an  uncommon  practice  for  livestock  raisers 
to  cross  certain  well-established  breeds  to  produce  crossed  animals 
to  feed  for  market. 

In  looking  over  the  reports  of  experiments  designed  to  test  the 
effects  of  crossing  in  both  wild  and  domesticated  animals  there  is 
little  disagreement  as  to  the  results  usually  obtained.  All  are 
practically  in  accord  that  crossing  diverse  breeds  or  races  of 
animals,  if  not  too  distantly  related,  may  frequently  result  in 
vigorous,  large  and  fertile  offspring,  excelling  either  parent  in 
one  or  more  respects.  For  example,  Castle  et  al  ('06)  find  that 
crossing  diverse  stocks  of  Drosophila  results  in  an  increase  in 
fertility  and  that  matings  between  different  inbred  lines  give 
progeny  with  increased  fertility  up  to  or  beyond  that  of  the  more 
fertile  parental  race.  In  Meriones  Bonhote  ('15)  states  that 
fertility  and  size  are  increased  by  crossing.  Castle  ('16)  has 
crossed  domesticated  races  of  guinea-pigs  with  the  wild  species 
from  Peru  with  the  result  that  there  is  a  noticeable  increase  in 
body  weight  over  either  pure  parent.  Gerschler  ('14)  crossed 
different  genera  of  fishes  and  obtained  large  increases  in  size  in 
the  first  hybrid  generation.  Xiphophorus  strigatus,  of  which  the 
males  were  43.0"  cm.  long  and  the  females  52.0  cm.,  when  crossed 
with  Platypoecilius  viaculatus,  of  which  the  males  were  26.0  and 
the  females  31.0  cm.  in  length,  gave  hybrid  males  54.0  cm.  and 
females  57.5  cm.      He  speaks  of  their  "gigantic  size." 

Fischer  ('13)  in  his  study  of  the  Rehoboth  hybrids,  a  race  in 
South  Africa  resulting  from  a  mixture  of  Hottentots  and  Boers, 
states  that  their  average  height  is  somewhat  greater  than  either 
the  Hottentots  or  the  Hollanders  and  South  Germans  of  whom 


INVESTIGATIONS   WITH    ANIMALS.  19 

statistics  are  available.  All  the  members  of  this  new  race  are  not 
first  generation  crosses  by  any  means,  but  they  are  not  many 
generations  removed  and  crossing  with  the  pure  Hottentots,  the 
shorter  parental  race,  is  frequent. 

When,  however,  the  literature  on  the  effects  of  inbreeding  in 
animals  is  examined  one  finds  the  greatest  diversity  of  facts  and 
opinions.  We  find  the  extreme  views  of  Kraemer  ('13)  who  states 
that  "continued  inbreeding  always  must  result  in  weakened  con- 
stitution, through  its  own  influence"  together  with  the  equally 
extreme  and  biased  opinion  of  Huth  ('75)  that  in  mankind  there 
is  no  injurious  effect  resulting  from  consanguineous  marriages 
which  cannot  be  accounted  for  on  other  grounds. 

Crampe  ('83),  Ritzema-Bos  ('94),  Guaita  ('98),  Fabre-Domengue 
('98)  and  Weismann  ('04)  by  inbreeding  mammals  and  birds 
found  that  the  process  was  accompanied  by  decreased  fertility, 
attended  more  or  less  commonly  by  lack  of  vigor,  diminution  in 
size,  and  pathological  malformations.  Castle,  Carpenter  et  al  ('06) 
inbreeding  extensively  the  fruit  fly,  Drosophila,  maintained  fer- 
tility by  selection,  so  that  at  the  end  of  59  generations  of  brother 
and  sister  matings  in  one  line  the  fecundity  was  no  less  at  the  end 
of  the  experiment  than  it  was  at  the  start.  There  was  some  indi- 
cation of  reduction  in  size  of  inbred  flies  when  compared  to  nor- 
mally crossed  stock  flies  reared  under  the  same  conditions.  Fur- 
thermore, fertility  was  increased  by  crosses  between  certain 
inbred,  lines  and  between  the  inbred  lines  and  stock  flies.  From 
this  fact  and  from  the  fact  that  their  experiments  show  that  the 
number  of  flies  in  a  brood  fluctuates  greatly,  due  to  temperature 
and  food  conditions,  it  is  not  positive  that  inbreeding  was  wholly 
without  injurious  effects.  It  is  evident  that  their  experiments  do 
show  clearly: 

1.  That  inbreeding  results  in  strains  of  unequal  fertility. 

2.  That  the  occurrence  of  absolute  sterility  was  pronounced  in 
the  first  part  of  the  experiment  with  the  "A"  line  but  almost 
entirely  disappeared  in  the  later  part  of  the  experiment.  The 
figures  as  I  have  calculated  them  from  their  table  I,  p.  736,  are 
as  follows: 

Percent  of  matings 
totally  sterile 

Generations 6  to  24  17 .  80 

25  to  42  18.47 

43  to  59  3.37 


20  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

This  result  is  to  be  expected  on  the  view  that  inbreeding  isolates 
homozygous  individuals  and  these  whenever  sterile  are,  of  course, 
eliminated. 

Moenkhaus  ('11)  and  Hyde  ('14)  by  similar  inbreeding  experi- 
ments with  Drosophila  have  also  found  that  sterility  is  increased 
in  the  first  stages  of  inbreeding  but  tends  to  be  eliminated  after 
this  process  is  long  continued.  Hyde  found  definite  evidence  that 
inbreeding  caused  reduction  in  size,  vigor,  rate  of  growth,  longevity 
and  fecundity  and  that  there  was  a  return  to  the  normal  condition 
on  crossing.  As  in  the  other  experiments  Hyde  found  that  selec- 
tion was  an  effective  agent  in  controlling  sterility. 

Both  Whitney  (12a)  and  A.  F.  Shull  (12a)  have  shown  that 
inbreeding  and  crossbreeding  have  considerable  effect  upon  the 
rotifer,  Hydatina  Senta,  in  the  size,  of  family,  number  of  eggs  laid 
per  day,  rate  of  growth  and  in  the  difficulty  of  rearing  the  animals. 

King  ('16)  has  obtained  results  with  albino  rats  which  are 
quite  in  agreement  with  those  of  Castle.  By  growing  about  one 
thousand  rats  in  each  inbred  generation,  and  selecting  the  best 
individuals  for  mating,  animals  have  been  carried  through  22 
generations  of  brother  and  sister  matings  without  loss  of  size, 
fertility,  longevity,  resistance  to  disease  and  with  constitutional 
vigor  unimpaired.    This  writer  states: 

"The  results  so  far  obtained  with  these  rats  indicate  that  close  inbreed- 
ing does  not  necessarily  lead  to  a  loss  of  size  or  constitutional  vigor  or  of 
fertility,  if  the  animals  so  mated  came  from  sound  stock  in  the  beginning 
and  sufficient  care  is  taken  to  breed  only  from  the  best  individuals." 

Here,  as  in  Drosophila,  inbreeding  isolates  diverse  types  of 
different  degrees  of  excellence.  In  this  case  individuals  are  ob- 
tained which  surpass  the  original  stock  before  inbreeding.  Thus 
we  have  "Goliaths"  among  inbred  rats  as  Darwin  found  "Heroes" 
in  morning-glories. 

Castle  ('16)  has  found  that  in  inbred  rats  "races  of  fair  vigor  and 
fecundity  can  be  maintained  under  these  conditions,  but  that  when  two 
of  these  inbred  races  are  crossed  with  each  other,  even  though  they  have 
their  origin  in  a  small  common  stock  many  generations  earlier,  an  imme- 
diate and  striking  increase  of  fecundity  occurs." 

The  evidence  from  relationship  marriages  in  human  stocks  is 
even  more  conflicting  and  conclusions  still  more  difficult  to  draw. 
Huth  ('75)  has  certainly  done  a  service  in  showing  that  consan- 


UNIVERSALITY   OF   HETEROSIS.  21 

guineous  marriages  seldom  result  in  the  disastrous  effects  usually 
attributed  to  them.  He  has  shown  that  incest  was  not  a  rare 
custom  and  that  races  which  have  undergone  such  practices  are 
many  of  them  far  from  weak.  Certainly,  races  have  practiced 
close  intermarriage  for  many  generations  with  no  marked  deterio- 
ration. The  Persians,  Spartans,  the  ruling  classes  among  the 
Egyptians  and  Polynesians  are  cited  by  Huth  in  support  of  this 
assertion.  The  data  from  human  matings,  however,  are  of  little 
value  since  the  close  unions  are  seldom  continued  many  genera- 
tions in  succession,  and  the  results  from  isolated  communities  mean 
little,  since  often  the  original  stock  is  exceedingly  diverse  so  as  to 
make  the  resulting  races  extremely  heterogeneous  in  hereditary 
constituents.  This  is  particularly  true  of  the  Rehoboths  and  the 
Pitcairn  Islanders  which  are  cited  as  instances  of  close  inter- 
marrying without  loss  of  racial  vigor. 

Looking  over  the  experiments  upon  animals  it  seems  as  unwise 
to  expect  that  inbreeding  may  not  have  some  deleterious  effects, 
which,  in  some  cases  at  least,  cannot  be  overcome  by  the  most 
rigid  selection,  as  it  is  to  hold  that  inbreeding  must  always  result 
injuriously.  It  is  to  be  expected  that  all  breeds  of  domestic 
animals  and  wild  species  will  not  be  equally  affected  by  inbreeding. 
Domesticated  animals  in  many  cases  are  more  widely  crossed  and 
diversified  than  wild  species,  and  those  characters  affected  by 
inbreeding  are  more  accentuated.  Certain  wild  species,  which, 
by  their  mode  of  life,  are  forced  to  endure  long  periods  of  isolation, 
and  consequently  more  or  less  close  inbreeding,  would  be  expected 
to  show  less  change  under  artificial  inbreeding.  Finally,  as  I 
shall  attempt  to  show  that  there  is  no  longer  a  question  as  to 
whether  or  not  inbreeding,  in  itself,  is  injurious,  the  effect  which 
inbreeding  will  have  on  any  organism  depends  solely  on  the 
hereditary  constitution  of  that  organism  at  the  time  the  inbreed- 
ing process  is  commenced. 

Universality  of  Heterosis. 

From  the  literature  on  the  subject  of  crossbreeding  it  is  to  be 
observed,  therefore,  that  the  occurrence  of  an  incentive  to  in- 
creased development  accompanying  germinal  heterogeneity  is 
widespread,  as  it  has  been  noted  in  plants  in  the  angiosperms, 
gymnosperms  and  pteridophytes,  and  according  to  Britton  ('98) 


22  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

there  is  even  some  slight  evidence  that  heterosis  occurs  in  the 
sporophyte  of  the  bryophytes. 

In  animals  the  mammals,  birds,  fishes,  insects  and  rotifers  show 
the  phenomenon  of  heterosis  although  in  some  of  the  unicellular 
animals,  as  we  shall  see  later,  the  evidence  is  not  so  clear. 

I  shall  now  take  up,  in  some  detail,  experiments  on  inbreeding 
and  crossbreeding  in  cultivated  plants,  principally  in  maize. 

A  Theoretical  Consideration  of  Inbreeding. 

Up  to  the  present  time  it  has  been  maintained  that  the  effects 
of  inbreeding  were  of  two  kinds,  an  isolation  of  homozygous 
biotypes  together  with  a  loss  of  a  physiological  stimulation  which 
was  considered  to  be  roughly  proportional  to  the  number  of  heter- 
ozygous allelomorphs  present  in  the  organism  at  any  time.  The 
reduction  of  the  number  of  heterozygous  allelomorphs  in  an  inbred 
population  is  automatic  and  varies  with  the  closeness  of  inbreeding. 

Pearl  ('15)  on  the  basis  of  the  number  of  ancestors  which  make 
up  the  pedigree  of  any  individual  has  worked  out  a  coefficient 
of  inbreeding  which  is  an  indication  of  the  degree  to  which  that 
individual  has  been  inbred.  The  fewer  the  number  of  ancestors 
the  greater  the  degree  of  inbreeding  which  may  vary  from  no 
inbreeding,  in  which  no  one  ancestor  appears  more  than  once 
in  the  pedigree  of  an  individual,  to  the  closest  kind  of  inbreeding 
in  which  no  more  than  one  ancestor  is  concerned  in  any  one 
generation  in  the  production  of  an  individual  (self-fertilization). 
The  latter  degree  is  only  approached  by  hermaphroditic  plants 
and  animals,  which  are  capable  of  self-fertilization  and  in  function- 
ally bisexual  animals  and  plants  by  brother  and  sister  matings. 
This  statement  of  inbreeding  must,  of  course,  leave  out  of  con- 
sideration any  germinal  change  which  might  take  place  by  means 
other  than  hybridization  and  as  Castle  ('16)  has  pointed  out  is 
modified  by  the  differences  in  heterozygosity  of  the  ancestors 
making  up  the  pedigree. 

The  automatic  reduction  in  the  number  of  heterozygous  allelo- 
morphic  pairs  in  an  inbred  population,  by  self-fertilization, 
follows  the  well  known  Mendelian  formula  by  which  any  hetero- 
zygous pair  forms  in  the  next  generation  50  percent  homozygotes 
and  50  percent  heterozygotes  in  respect  to  that  pair.  Since  the 
homozygotes  must  always  remain  homozygous  and  the  hetero- 
zygotes are  halved  each  time  and  one  half  added  to  the  homo- 


A  THEORETICAL  CONSIDERATION  OF  INBREEDING. 


23 


zygotes  the  reduction  in  the  number  of  heterozygous  elements 
proceeds  as  a  variable  approaching  a  limit  by  one  half  the  differ- 
ence in  each  generation.  The  curve  illustrating  this  condition 
is  shown  as  No.   1  in  Fig.  I.      Various  formulae  dealing  with 


100$ 


Percent  of  Heterozygous 
Individuals  in  Each  Selfed 
Generation  when  the  Number 
of  Allelomorphs  Concerned 
Are:  1,5,10,15. 


10 


Segregating  Generations 

Figure  I.  The  percent  of  heterozygous  individuals  and  the  percent  of 
heterozygous  allelomorphic  pairs  in  the  whole  population  in  each 
generation  of  self-fertilization. 


inbreeding  have  been  discussed  by  East  and  Hayes  ('12),  Jennings 
('12,  '16),  Pearl  ('15)  and  Bruce  ('17). 

It  should  be  remembered  that  this  reduction  applies  only  to 
the  whole  population  in  which  every  member  is  inbred  and  all 


24  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

the  progeny  grown  in  every  generation.  In  practice,  in  an 
inbreeding  experiment,  only  one  individual  in  self-fertilization  or 
two  individuals  in  brother  and  sister  matings  are  used  to  produce 
the  next  generation.  Thus  the  rate  at  which  complete  homo- 
zygosis  is  approached  depends  on  the  heterozygosity  of  the 
individuals  chosen.  Theoretically  in  any  inbred  generation  the 
progenitors  of  the  next  generation  may  either  be  completely 
homozygous  or  completely  heterozygous  or  any  degrees  in  between 
depending  upon  chance.  The  only  condition  which  must  follow 
in  self-fertilization  is  that  no  individual  can  ever  be  more  hetero- 
zygous than  its  parent  but  may  be  the  same  or  less.  Thus  it  is 
seen  that  inbreeding,  as  it  is  practiced,  may  theoretically  never 
cause  any  reduction  in  heterozygosity,  or  it  may  bring  about 
complete  homozygosity  in  the  first  inbred  generation.  In  other 
words  the  rate  at  which  homozygosity  is  approached  may  vary 
greatly  in  different  lines.  However,  as  the  number  of  heter- 
ozygous factors  at  the  commencement  of  inbreeding  increases  the 
more  nearly  will  the  reduction  to  homozygosity  follow  the  curve 
shown  because  the  chance  of  choosing  a  completely  homozygous 
or  completely  heterozygous  individual  in  the  first  generations 
will  become  less. 

In  Table  1  is  shown  the  theoretical  classification  of  the  progeny 
of  a  self-fertilized  organism  which  was  heterozygous  with  respect 
to  15  independent  mendelizing  units.  .  It  will  be  seen  that  the 
bulk  of  the  individuals  lie  between  classes  6  and  11  where  none  of 
the  members  are  heterozygous  for  more  than  10  factors  nor  less 
than  5.  In  other  words  any  individual  selected  for  the  progenitor 
of  the  next  generation  would  probably  come  from  the  middle 
classes  and  therefore  it  would  be  heterozygous  for  about  half 
the  factors  that  its  parent  was.  The  chance  that  this  individual 
would  not  come  from  the  mid-classes  between  6  and  11  would  be 
about  1  out  of  10.  The  chance  that  it  would  be  completely  homo- 
zygous or  completely  heterozygous  would  be  1  out  of  32,768.  If 
20  instead  of  15  factors  were  concerned  the  chances  would  be 
1  out  of  1,048,576. 

This  condition  by  which  the  progenitor  of  each  generation  tends 
to  be  half  as  heterozygous  as  its  parent  holds  true  for  any  number 
of  factors  and  in  every  generation.  Also  in  Table  1  it  can  be  seen 
that  the  progeny  as  a  whole  has  an  equal  number  of  heterozygous 
factor  pairs   as   homozygous   factor   pairs  in  respect   to   those 


A  THEORETICAL  CONSIDERATION  OF  INBREEDING. 


25 


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26  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

characters  in  which  the  parent  was  heterozygous.  So  it  is  that 
in  practice  the  reduction  in  growth  accompanying  inbreeding 
(on  the  assumption  that  heterosis  is  correlated  with  heterozygosity) 
is  greatest  at  first,  rapidly  becomes  less  and  finally  ceases  for  all 
practical  purposes. 

If  there  were  no  deviating  factors  the  curve  of  reduction  should, 
in  the  majority  of  cases,  approximate  curve  1  in  Fig.  I.  However, 
it  has  never  been  assumed  that  the  amount  of  heterosis  was 
perfectly  correlated  with  the  number  of  heterozygous  factors. 
Moreover,  since  the  heterozygous  individuals  are  more  vigorous 
than  the  homozygous,  selection,  either  unconscious  or  purposeful, 
would  favor  the  more  heterozygous  so  that  the  tendency  might 
be  that  the  actual  approach  to  homozygosity  would  not  proceed 
at  as  fast  a  rate  as  the  theoretical  curve  would  indicate. 

Self-fertilization  is  the  quickest  and  surest  means  of  obtaining 
complete  homozygosis  for  the  reason  that  whenever  any  pair  of 
allelomorphs  becomes  homozygous  it  must  always  remain  so  long 
as  self-fertilization  takes  place,  whereas  in  brother  and  sister 
mating  a  homozygote  may  be  mated  to  a  heterozygote.  Thus 
we  see  from  Jennings'  ('16)  tables  that  6  generations  of  self- 
fertilization  are  more  effective  than  17  generations  of  brother 
and  sister  matings  in  bringing  about  homozygosis.  The  reduction 
in  heterozygous  allelomorphs  in  a  population  as  a  whole  follows 
curve  1  in  Fig  I  irrespective  of  the  number  of  factors  concerned, 
provided,  as  stated  before,  that  a  random  sample  of  all  the  different 
classes  of  individuals  are  selfed  and  used  as  progenitors  for  the 
next  generation  and  that  there  is  equal  productiveness  and  equal 
viability.  If  the  heterozygotes  are  more  productive,  as  in  many 
cases  they  are,  the  reduction  to  complete  homozygosity  will  be 
delayed. 

The  number  of  completely  homozygous  individuals  in  any  gener- 
ation, inbred  by  self-fertilization,  differs  according  to  the  number 
of  heterozygous  factors  concerned  at  the  time  that  the  inbreeding 
process  is  commenced.  The  curves  showing  the  reduction  in  the 
number  of  individuals  heterozygous  in  any  factors,  where,  1,  5, 
10  and  15  factors  are  concerned  at  the  start  are  given  in  Fig.  I 
calculated  from  the  formula  given  by  East  and  Hayes  ('12). 
The  curve  for  the  reduction  in  heterozygous  individuals,  where 
one  factor  only  is  concerned,  is  identical  with  the  curve  showing 
the  reduction  in  heterozygous  factors  in  an  inbred  population 


RESULTS    OF   INBREEDING.  27 

where  any  number  of  factors  are  concerned.  In  any  case  almost 
complete  homozygosity  is  reached  in  about  the  tenth  generation 
on  the  average,  although  theoretically  it  may  be  reached  in  the 
first  generation,  or  may  never  be  reached  when  a  single  individual 
is  used  in  each  generation  to  perpetuate  the  line. 

Assuming,  then,  that  the  loss  of  the  stimulation,  accompanying 
heterozygosity,  is  correlated  with  the  reduction  in  the  number 
of  heterozygous  allelomorphs  we  should  expect  to  find  the  decrease 
of  heterosis  greatest  in  the  first  generations,  rapidly  becoming 
less  until  no  further  loss  is  noticeable  in  any  number  of  subsequent 
generations  of  inbreeding,  and  that,  on  the  average,  the  loss  will 
become  negligible  at  about  the  eighth  generation  and  from  then 
on  no  further  marked  change  will  take  place.  Some  cases  are 
to  be  expected  in  which  stability  is  reached  before  this  generation 
and  some  cases  in  which  it  is  not  reached  until  later  or  may  even 
theoretically  never  be  reached.  With  these  assumptions  in  mind 
let  us  see  what  are  the  actual  results  of  long  continued  inbreeding 
in  maize. 

The  Results  of  Inbreeding  the  Naturally  Cross-pollinated 

Maize  Plant. 

The  behavior  of  maize  during  six  generations  of  inbreeding  by 
self-fertilization  has  already  been  reported  by  East  and  Hayes 
('12).  The  same  inbred  strains  have  been  continued  and  in  some 
cases  the  results  up  to  the  eleventh  generation  are  given  here. 

In  the  previous  publication  it  was  stated  that  a  loss  of  vege- 
'tative  vigor  has  followed  every  case  of  inbreeding  in  maize. 
Some  plants  had  been  obtained  which  were  unable  to  reproduce 
themselves.  Those  strains  which  were  maintained  became  uni- 
form but  differed  considerably  from  each  other.  It  was  con- 
sidered at  the  end  of  the  period  of  inbreeding  that  some  strains 
were  appreciably  better  than  others  in  their  ability  to  yield. 
Six  additional  years  of  inbreeding  with  this  material  has  confirmed, 
in  the  main,  these  conclusions.  A  further  appreciable  reduction 
in  productiveness,  however,  has  taken  place  in  all  lines  together 
with  certain  changes  in  various  parts  of  the  plants. 

The  original  experiment  began  with  four  individual  plants 
obtained  from  seed  of  a  commercial  variety  of  Learning  dent 
corn  grown  in  Illinois.     This  variety  was  given  the  number  1 


28  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

and  the  four  plants  which  were  self-pollinated  and  selected  for 
continuation  of  the  inbreeding  experiment  were  numbered  1-6, 
1-7,  1-9  and  1-12.  These  four  strains  were  continued  each  year 
by  self-pollination.  In  the  second  inbred  generation  two  self- 
pollinated  plants  in  the  1-7  line  were  saved  for  seed  and  from  them 
two  inbred  lines  were  split  off  which  therefore  came  originally 
from  one  line  inbred  two  generations.  These  are  numbered 
1-7-1-1  etc,  and  1-7-1-2  etc.  In  a  similar  way  these,  and  the  other 
inbred  lines,  were  further  split  up  in  subsequent  generations. 
After  the  experiment  was  started  with  the  dent  corn  inbreeding 
was  commenced  with  other  material.  Two  inbred  strains  of 
floury  corn,  Nos.  10-3  and  10-4,  originally  from  the  same  variety, 
have  been  maintained  and  also  two  strains  of  flint,  Nos.  5  and  29, 
and  two  strains  of  popcorn  Nos.  64  and  65.  Chief  attention  has 
been  paid  to  the  inbred  strains  of  Learning  corn  (the  longest 
inbred)  and  most  of  the  data  presented  here  have  resulted  from 
this  material.  Many  other  varieties  besides  these  have  been 
inbred  for  many  generations  in  connection  with  other  investi- 
gations and  while  they  are  not  specifically  mentioned  the  observa- 
tions as  a  whole  include  these. 

In  Tables  2  and  3  the  yield  and  height  of  some  of  these  inbred 
strains  are  given.  In  1916  seed  of  the  original  Learning  variety 
was  obtained  which  had  been  grown  in  the  meantime  in  the  same 
locality  whence  it  was  originally  secured  and  was  grown  for 
comparison  with  the  inbred  strains.  This  variety  in  Illinois  in 
1905  yielded  at  the  rate  of  88  bushels  per  acre,  and  in  Connecticut 
in  1916  at  the  rate  of  74.7  bushels.  While  there  is  no  proof  that 
any  change  has  not  taken  place  in  the  original  variety  there  is  no 
reason  to  suppose  that  it  has  changed  to  any  great  extent.  Grown 
under  the  same  conditions  in  1916  the  four  inbred  Learning 
strains  yielded  from  one-third  to  one-half  as  much  as  the  original 
non-inbred  variety. 

With  regard  to  rate  of  reduction  in  yield  or  the  constancy  of 
the  varieties  during  the  later  generations  it  is  difficult  to  draw 
conclusions  from  these  figures  owing  to  the  fluctuation  in  yield 
from  year  to  year  due  to  seasonal  conditions  and  to  the  difficulty 
of  accurate  testing  in  field  plot  work,  which  is  recognized  by  all 
who  have  made  such  tests.  As  was  stated  in  the  first  report  the 
yields  for  1909  were  too  low  and  in  1911  much  too  low  on  account 
of  poor  seasons.     No  yields  were  taken  on  any  of  the  strains  in 


RESULTS   OF   INBREEDING. 


29 


Table  2. 

The  effect  of 

INBREEDING  on  the  yield  and  height  of  maize. 

Year 
grown 

No.  of 

genera- 
tions 
selfed 

Four  inbred  strains  derived  from  a  variety  of  Learning  dent  corn. 

1-6-1-3-eto. 

1-7-1-1-eto. 

1-7-1-2-etc. 

1-9-1-2 

-etc. 

Yield 

bu.  per 

acre 

Height 
inches 

Yield 

bu.  per 

acre 

Height 
inches 

Yield 

bu.  per 

acre 

Height 
inches 

Yield 

bu.  per 

acre 

Height 
inches 

1916 
1905 
1906 
1908 
1909 
1910 
1911 
1912 
1913 
1914 
1915 
1916 
1917 

0 
0 

1 

2 
3 
4 
5 
6 
7 
8 
9 
10 
11 

74.7 
88.0 
59.1 
95.2 
57.9 
80.0 
27:7 

117.3 

86.7 

74.7 

88.0 

60.9 

190759 . 3 

190846  . 0 

63.2 
25.4 

111.3 

81.1 

74.7 

88.0 

60.9 

190759 . 3 

190859  . 7 

68.1 
41.3 

117.3 

90.5 

74.7 
88.0 
42.3 
51.7 
35.4 
47.7 
26.0 
191338.9 
191445  _  4 

191521.6 

19l630.6 

191731.8 

117 

3 

76 

5 

41.8 
78.8 
25.5 
32.8 
46.2 

96.0 

97.7 
103.7 

39.4 
47.2 
24.8 
32.7 
42.3 

85 

0 

83.5 

58.5 

88.0 

78 
82 

7 

84.9 

78.6 

19.2 
37.6 

86.9 
83.8 

4 

Table  3.     The  effect  of  inbreeding  on  the  yield  of  maize. 


Two  inbred  strains  of 

One  inbred  strain  of 

floury  corn 

flint  corn 

Year 

No.  of 
genera- 

10-3-7-ete. 

10-4-S-etc. 

5-8-6-etc. 

grown 

tion's 

Yield 

Yield 

Yield 

"          selfed 

bu.  per 

bu.  per 

bu.  per 

acre 

acre 

acre 

1908 

0 

70.5 

70.5 

75.7 

1909 

1 

56.0 

43.0 

47.5 

1910 

2 

67.0 

48.7 

36.1 

1911 

3 

39.1 

29.3 

11.5 

1912 

4 

1913 

5 

32.2 

49.5 

30.4 

1914 

6 

52.6 

38.1 

1915 

7 

1916 

8 

13.9 

16.6 

18.3 

1917 

9 

26.6 

24.0 

30  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

1912.  The  yields  in  1914  are  too  high  and  in  1915  too  low  for  the- 
same  reasons.  Also  in  1915  the  yields  are  unreliable  because 
only  a  few  plants  were  available  to  calculate  yields  from  as  most 
of  them  were  used  for  hand  pollination.  During  the  last  three 
years  of  the  test  samples  of  corn  have  been  dried  to  a  uniform 
moisture  basis  and  the  yields  calculated  to  bushels  of  shelled 
corn  per  acre  with  12  per  cent,  moisture.  This  has  probably 
had  a  tendency  to  reduce  the  yields  somewhat  as  these  inbred 
strains  are  very  late  in  maturing  and  consequently  contain  large 
amounts  of  water. 

With  these  points  in  mind  an  examination  of  the  table  shows 
that  from  the  beginning  of  the  experiment  to  the  ninth  generation 
there  has  been  a  tremendous  drop  in  productiveness,  so  that  in 
that  generation  the  strains  are  approximately  only  one-third  as 
productive  as  the  variety  before  inbreeding.  From  the  ninth 
to  the  eleventh  generation  there  has  been  at  least  no  reduction 
in  productiveness,  and  practically  no  change  in  visible  plant  or 
ear  characters. 

In  the  previous  publication  it  was  stated  (U.  S.  Dept.  of  Agric, 
B.  P.  I.  Bull.  243,  pp.  23-24)  that 

"  strain  No.  6,  is  a  remarkably  good  variety  of  corn  even 
after  five  generations  of  inbreeding.  It  yielded  eighty  bushels  per  acre- 
in  1910.  The  yield  was  low  in  1911,  but  since  all  yields  were  low  that 
year  it  can  hardly  be  doubted  that  this  strain  will  continue  to  produce 

good  normal  yields  of  grain The  poorest  strain,  No.  12,  is  partially 

sterile,  never  fills  out  at  the  tip  of  the  ear  and  can  hardly  exist  alone. 
In  1911  it  yielded  scarcely  any  corn  but  will  no  doubt  continue  its  exist- 
ence as  a  partly  sterile  variety." 

These  statements  will  have  to  be  modified  somewhat.  Although 
No.  6  is,  in  the  eleventh  generation,  still  the  most  vigorous  inbred 
strain,  as  a  producer  of  grain,  however,  it  can  hardly  be  considered 
to  give  "  good  normal  yields."  The  plants,  nevertheless,  are 
perfectly  healthy  and  functionally  normal  in  every  way  except 
for  an  extreme  reduction  in  the  amount  of  pollen  which  they 
produce.  The  strain  No.  12  was  lost.  Since  the  difficulty  of 
carrying  along  any  inbred  strain  is  very  great  owing  to  failure 
to  pollinate  at  the  right  time,  attacks  of  fungus  on  the  ear  enclosed 
in  a  paper  bag,  and  poor  germination  in  the  cold,  wet  weather 
common  in  New  England  at  corn  planting  time,  the  loss  of  this 
strain  might  be  easily  accounted  for  without  supposing  that  it 


RESULTS    OF   INBREEDING.  31 

-simply  ran  out.  It  may  be  that  this  strain  could  have  been 
perpetuated  if  sufficient  effort  had  been  put  forth  to  do  so.  In 
view  of  the  further  reduction  in  the  other  strains,  however,  the 
maintaining  of.  this  strain  would  have  been  extremely  difficult. 

Complete  records  on  the  height  of  plant  are  wanting  for  many 
of  the  generations,  and,  unfortunately,  in  the  first  part  of  the 
inbreeding  period.  What  figures  are  available  certainly  show 
that  very  little  change  in  height  has  taken  place  in  all  four  strains 
during  the  last  seven  generations.  Strain  No.  6  has  increased 
in  height,  if  anything.  Height  is  less  affected  by  environmental 
factors  than  is  yield  and  in  that  respect  is  a  more  reliable  indicator. 
However,  great  changes  in  the  structure,  size  and  productiveness 
may  take  place  without  height  of  plant  being  greatly  altered.  , 

From  the  figures  given  in  Table  2  there  is  some  evidence  that 
these  strains  have  reached  about  the  limit  of  reduction  in  pro- 
ductiveness and  that  there  has  been  very  little  change  in  the  last 
three  years.  This,  however,  is  not  proven.  The  continuation 
of  inbreeding  is  necessary  for  conclusive  evidence  on  this  point. 
As  the  crosses  between  individual  plants  within  these  inbred 
strains  have  given  very  little  increase  over  the  selfed  strains, 
as  will  be  shown  later,  and  from  the  fact  that  almost  no  visible 
change  has  taken  place  in  these  four  strains  during  the  past  three 
years  that  I  have  had  them  under  observation,  it  seems  apparent 
to  me  that  the  reduction  in  vegetative  vigor  and  productiveness 
is  very  nearly  at  an  end. 

Jn  Tables  4,  5,  6  and  7  are  given  the  frequency  distributions  of 
height,  length  of  ear,  number  of  nodes  and  the  number  of  rows 
of  grain  on  the  cob  of  the  original,  non-inbred  Learning  variety 
and  several  inbred  strains  derived  "from  this  variety  after  nine 
or  ten  generations  of  selfing.  All  the  plants  from  which  the  data 
were  taken  were  grown  on  the  same  field  in  the  same  year.  Four 
different  plots  of  the  variety  were  grown  in  different  parts  of  the 
field  and  the  data  on  these  plots  are  given  separately  and  totaled 
in  the  tables.  It  can  be  seen  from  these  that.no  great  variations 
in  range,  mean,  standard  deviation  or  coefficient  of  variability 
were  caused  by  environmental  factors.  The  pedigree  numbers 
show  the  relationship  of  the  several  inbred  strains  to  each  other. 

From  these  tables  it  can  be  seen  that  both  height  of  plant 
and  length  of  ear  have  been  reduced,  but  in  different  degrees 
in  different  lines.  In  some  strains  reduction  in  height  amounts 
to  40  inches  and  in  length  of  ear  to  3.5  inches.    The  reduction  in 


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RESULTS    OF   INBREEDING. 


33 


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RESULTS    OF   INBREEDING. 


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36  CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 

length  of  ear  is  even  more  than  it  seems  from  this  table  because 
the  variety  contained  plants  which  produced  two  ears  of  which 
the  second  is  usually  smaller  than  the  first;  whereas  the  inbred 
strains  almost  never  produce  more  than  one  ear  to  a  plant. 

The  number  of  n6des  per  plant  is  reduced  but  as  compared  to 
height  and  length  of  ear  this  reduction  is  very  much  less.  In 
the  number  of  rows  of  grain  on  the  cob  there  is  a  reduction  in 
some  lines  and  an  increase  in  others.  These  tables  show  in  the 
clearest  manner  that  inbreeding  has  a  greater  effect  on  some 
characters  than  on  others,  and  that  segregation  of  characters 
has  occurred.  Perhaps  the  most  noticeable  effect  of  inbreeding 
as  shown  by  these  tables  is  the  reduction  in  variability  as  brought 
out  by  the  range  and  statistical  constants.  This  reduction  in 
variability  is  most  apparent  in  the  characters  which  are  the  least 
reduced  by  inbreeding — number  of  nodes  and  number  of  rows 
of  grain  on  the  ear — although  the  low  variability  is  also  apparent 
in  height  and  length  of  ear.  In  variability,  also,  there  is  a  difference 
between  different  lines. 

The  variability  in  height  and  length  of  ear  of  the  inbred  strains 
is  higher  than  it  should  be,  owing  to  the  fact  that  it  was  difficult 
to  obtain  a  perfect  stand  of  plants,  on  account  of  poor  germina- 
tion of  the  seeds  of  the  inbred  strains.  The  aim  was  to  have 
three  plants  in  a  hill.  From  four  to  eight  seeds  were  planted  as 
far  as  a  limited  supply  of  seed  would  permit,  and  later,  thinned 
to  three  plants.  In  spite  of  this  precaution  it  was  extremely 
difficult  to  get  anything  like  a  perfect  stand,  so  missing  plants 
were  replanted  as  soon  as  possible.  These  replants,  owing  to 
their  late  start,  never  entirely  caught  up  with  the  other  plants 
and  are  shorter  in  height  and  have  smaller  ears  in  consequence. 
It  is  unfortunate  that  this  practice  was  followed  because  it  is 
believed  that  much  more  reliable  results  would  have  been  obtained 
otherwise.  On  the  other  hand  missing  plants  introduce  another 
source  of  error — that  of  unequal  opportunity  to  grow.  Because 
there  was  abundant  seed  of  the  variety,  and  it  germinated  well, 
practically  complete  stands  of  these  plants  were  obtained. 

The  reduction  in  variability  is  more  apparent  in  the  details 
of  the  structure  of  the  plants  and  ears  which  cannot  be  expressed 
statistically.  The  beautiful  uniformity  of  these  plants  in  all 
characteristics  at  the  present  time  is  one  of  their  most  striking 
features.  This  can  be  seen  fairly  well  in  the  accompanying 
photographs.     (Plates  I  to  V). 


RESULTS    OF   INBREEDING.  37 

In  view  of  this  fact  of  great  uniformity  and  constancy  as  a 
result  of  inbreeding  one  is  astonished  at  the  statement  made 
recently  by  Stout  ('16)  in  a  discussion  of  the  results  obtained 
from  inbreeding  in  maize  by  East  and  Hayes.  Stout  says  (pp. 
420-421) :       ' 

"strains  similar  in  homozygosity  show  widest  variation  indicative  of 
spontaneous  variation  in  natural  vigor  which  is  suggested  that  in  such 
highly  cultivated  varieties  such  as  corn  extreme  sporadic  variations  may 
be  constantly  occurring,  a  condition  which  is  well  sho.^n  by  the  numerous 
and  well-known  results  of  the  ear  to  row  test." 

Several  curious  misconceptions  are  to  be  noted  in  this  statement. 
In  the  first  place,  it  has  never  been  maintained  by  anyone  to  my 
knowledge  that  an  equal  number  of  generations  of  inbreeding  produce 
an  equal  amount  of  homozgosity  in  different  lines.  Secondly,  it  has 
never  been  proposed  that  the  degree  of  heterozygosity  determined 
the  form  or  structure  of  any  organism,  but  that  such  a  condition 
was  accompanied  by  a  stimulus  to  development  which  merely 
increased  the  expression  of  many  hereditary  factors.  This  stimulus 
is  considered  to  be  without  any  great  effect  in  itself  on  variability. 
Granted  that  the  inbred  strains  were  equal  in  homozygosity  at 
that  time,  that  was  no  reason  why  they  should  be  similar  in 
vigor  or  in  any  other  respect- — in  fact  the  expectation  is  exactly 
the  reverse  of  this.  With  regard  to  "spontaneous"  and  "sporadic" 
variation  these  inbred  strains  show  unmistakably  that  there  is 
practically  no  sporadic  or  spontaneous  variation,  that  the  indi- 
viduals making  up  an  inbred  strain  are  remarkably  constant  and 
uniform  after  some  degree  of  homozygosity  is  obtained  and  that 
the  diversity  between  different  lines  can  be  perfectly  accounted 
for  on  the  basis  of  segregation  of  characters.  Also,  in  the  following 
paragraphs  in  his  paper  Stout  fails  to  see  the  distinction  between 
crosses  of  diverse  inbred  lines  and  between  crosses  of  non-inbred 
commercial  varieties.  Because  Collins  ('14)  and  Hayes  ('14) 
failed  to  obtain  increases  in  all  crosses  between  commercial  vari- 
eties of  similar  type  Stout  would  question  whether  crossing  in 
maize  was  ever  beneficial.  It  is  quite  to  be  expected  that  there 
are  many  varieties  already  so  widely  crossed  that  further  crossing 
does  not  result  in  greater  heterozygosity,  but  may  even  reduce  it. 
It  is  only  in  crosses  between  somewhat  different  varieties,  like 
flint  and  dent  (Jones  and  Hayes  '17)  or  between  varieties  from 


38  CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 

different  geographical  regions  (Collins  '10)  that  any  great  amount 
of  heterosis  in  naturally  widely  crossed  varieties  is  to  be  expected. 

Although  there  has  been  a  striking  reduction  in  size  of  plant, 
general  vegetative  vigor  and  productiveness  in  these  inbred 
strains  of  maize,  and  in  comparison  with  non-inbred  varieties  the 
inbred  plants  are  more  difficult  to  grow,  emphasis  must  be  put  on 
the  fact  that  the  plants  are  normal  and  healthy.  The  monstrosities 
which  are  common  in  every  field  of  maize,  such  as  the  occurrence 
of  seeds  in  the  tassels,  anthers  in  the  ears,  dwarf  plants,  completely 
sterile  plants,  mosaic  and  albino  plants  and  other  similar  anomalies 
never  appear  in  these  inbred  strains.  Furthermore,  in  the  details 
of  the  size,  shape,  structure  and  position  of  the  tassels,  leaves, 
stalks  and  ears,  these  inbred  strains  show  the  most  striking  uni- 
formity. These  minor  details  which  characterize  each  of  these 
groups  of  plants  are  difficult  to  describe  but  are  perhaps  the  most 
noticeable  feature  about  them.  The  stalks,  the  tassels  or  the  ears 
of  all  of  these  four  Learning  strains  if  mixed  together  could  be 
separated  without  the  slightest  difficulty  by  anyone  familiar  with 
them.  Some  of  the  differences  which  characterize  the  ears  of 
these  four  strains  are  shown  in  Plate  lb.  It  is  to  be  noticed  in 
this  photograph  that  Nos.  1-7-1-2  and  1-7-1-1,  which  were 
originally  from  the  same  line,  both  have  flat  cobs.  In  one  of  them, 
however,  it  is  colored,  in  the  other  uncolored.  Other  differences 
are  to  be  seen  in  shape  and  color  of  seeds. 

The  segregation  of  row  number  accompanied  by  a  reduction  in 
variability  in  these  two  strains  is  shown  in  Table  8  and  Fig  II. 
Data  previous  to  the  third  generation  are  not  available  but  since 
then  a  noticeable  change  in  average  row  number  has  taken  place 
without  any  selection  one  way  or  the  other.  The  variability  of 
each  line  has  decreased  at  the  same  time.  Whether  the  increase 
in  variability,  after  the  eighth  generation,  has  any  significance 
is  not  known.  It  is  possibly  due  to  the  fact  that  both  lines  have 
become  irregular  in  row  number  so  that  the  correct  determination 
of  the  row  number  has  been  rendered  more  difficult  in  the  later 
generations.  Also  the  number  of  plants  grown  in  the  generations 
from  the  7th  to  the  10th  are  much  too  few  to  base  accurate  con- 
clusions upon.  The  sharp  increase  in  average  row  number  and 
decrease  in  variability  in  the  8th  generation  are  probably  due  to 
the  unusually  favorable  growing  conditions  of  that  year. 


KESULTS   OF   INBREEDING. 


39 


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CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 


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Generations  Inbred 


Figure  II.     The  reduction  in  variability  and  segregation  of  number  of 
rows  of  grain  on  the  ear  in  selfed  strains  of  maize. 


East  and  Hayes  ('12)  have  noted  many  characters  which  are 
isolated  from  maize  by  inbreeding.  In  addition  to  these,  several 
other  characters  have  been  isolated  in  this  and  in  other  material. 
One  of  these  characters  is  a  constant  difference  in  shade  of  color 
of  the  foliage — some  are  dark  green,  others  are  light,  yellowish 
green.  Some  strains  are  lacking  in  root  development  and  never 
stand  upright  throughout  the  season.  Some  have  a  single-stalked 
unbranched  tassel,  while  others  are  profusely  branched.  Some 
strains  have'  peculiarly  wrinkled  or  wavy  leaves,  particularly 
noticeable  in  the  first  leaves.  Some  strains  produce  a  small  pro- 
portion of  connate  seeds  similar  to  those  observed  by  Kempon 
('13)  in  nearly  every  ear,  while  their  occurrence  has  never  been 
observed  on  other  inbred  lines  derived  from  the  same  source. 
There  are  also  marked  differences  in  susceptibility  to  disease  as 
will  be  shown  later. 

These  illustrations  are  sufficient  to  demonstrate  beyond  doubt 
that  by  far  the  greatest  amount  of  the  fluctuating  variability 
found  among  ordinary  cross-fertilized  plants  is  due  to  the  segrega- 
tion and  re-combination  of  definite  and  constant  hereditary 
factors.  Many  of  these  characters  are  seldom  seen  in  continually 
cross-pollinated  plants,  and  never  are  so  many  combined  together. 


RESULTS    OF   INBREEDING.  41 

This  is  due  to  the  fact  that  they  are  recessive  in  nature  and  com- 
plex in  mode  of  inheritance.  The  most  significant  feature  about 
the  characters  which  make  their  appearance  in  inbred  strains  is 
that  none  of  them  can  be  directly  attributed  to  a  loss  of  a  physio- 
logical stimulation,  although  undoubtedly  many  of  them  may  be 
modified  by  the  vigor  of  the  plant  upon  which  they  are  borne. 
There  is  no  one  specific  character  common  to  all  inbred  strains 
but  simply  a  general  loss  of  vigor,  a  general  loss  of  size  and  of 
productiveness  accompanied  by  the  appearance  of  specific  char- 
acters more  or  less  unfavorable  to  the  plants'  best  development 
but  these  unfavorable  characters  are  never  all  found  in  one  inbred 
strain,  nor  is  any  one  character  common  to  all  inbred  strains. 

Probably  the  most  common  result  of  inbreeding  in  maize  is  a 
reduction  in  the  amount  of  pollen  produced.  This  becomes  appar- 
ent in  a  smaller  size  of  all  parts  of  the  tassel,  in  shrunken  and 
abortive  anthers  which  are  often  never  released  by  the  glumes, 
with  a  consequent  reduction  in  the  amount  of  pollen  available  for 
fertilization.  A  normal  corn  plant  should  produce,  on  the  average, 
anywhere  from  lcc.  to  lOcc.  or,  in  some  cases,  very  much  more 
pollen.  I  have  made  no  actual  measurements  of  the  amounts  pro- 
duced. Many  of  the  inbred  strains,  however,  now.  produce  only 
a  small  fraction  of  a  cubic  centimeter  of  pollen,  and  the  production 
of  this  small  amount  is  much  affected  by  weather  conditions,  so 
that  many  strains,  otherwise  well  developed  and  productive,  are 
maintained  with  the  utmost  difficulty. 

It  has  been  my  experience  that  self -sterility  in  corn  is  due  to 
ovule  or  pollen  abortion.  Whenever  pollen  is  obtained  it  seems 
to  be  able  to  function.  Failures  to  obtain  seed  after  pollen  is  ap- 
plied are  common,  but  are  usually  attributed  to  external  factors. 
At  least  I  know  of  no  clear  case  where  pollen  is  produced  in  which 
it  fails  to  fertilize  the  ovules  of  plants  which  were  capable  of  being 
fertilized  by  other  pollen.  Many  cases  of  complete  abortion  of 
the  pistillate  part  of  the  plant  must  occur,  as  many  plants  are 
lost  through  failure  to  set  seed  when  good  pollen  has  been  applied. 
Just  where  the  trouble  lies  is  not  always  possible  to  detect.  Un- 
doubtedly, many  cases  of  complete  abortion  of  either  staminate 
or  pistillate  functions,  or  both,  occur  during  inbreeding,  and  the 
plants  are  eliminated  for  that  reason. 

Reduction  in  the  amount  of  pollen  produced  is  less  serious  than 
a  reduction  in  the  number  of  ovules,  as  a  very  small  amount  of 


42  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

pollen  suffices  for  fertilization  when  conditions  are  right.  For  that 
reason  unconscious  selection  for  good  ovule  production  has  been 
much  more  rigid  than  for  pollen  production.  That  is  the  reason, 
I  believe,  that  more  inbred  strains  now  show  a  greater  reduction 
in  the  staminate  function  than  in  the  pistillate. 

A  significant  feature  of  the  effect  of  inbreeding  upon  sterility  is 
that  some  inbred  strains  are  perfectly  normal  in  their  production 
of  pollen,  and  the  amount  of  pollen  produced  is  only  a  little  less 
than  non-inbred  plants,  owing  to  the  reduced  vigor  and  size  of 
the  plants  which  produce  the  tassels.  Out  of  about  twenty-five 
inbred  strains  carried  through  at  least  seven  generations,  three  of 
them  are  perfectly  normal  in  the  structure  and  function  of  their 
staminate  parts.  One  of  the  Learning  strains  (No.  1-9)  produces 
more  pollen  than  many  non-inbred  varieties  growing  nearby.  In 
every  case,  however,  those  plants  which -produce  the  best  devel- 
oped ears  are  the  poorest  producers  of  pollen,  and  those  strains 
which  produce  abundant  pollen  have  ears  which  are  poorly  de- 
veloped. In  other  words,  inbreeding  is  bringing  about  a  tendency 
for  maize  to  change  from  a  functionally  monoecious  plant  to  a 
functionally  dioecious  plant  although,  morphologically,  both 
staminate  and  pistillate  parts  are  still  present.  This  is  illustrated 
in  Plates  VI,  a  and  b,  where  tassels  and  ears  of  four  of  the  inbred 
strains  are  shown. 

Although  no  systematic  selection  has  been  practiced  throughout 
the  inbreeding  experiment  a  great  deal  of  selection  upon  many 
characters  has  been  unavoidable  as  it  is  unavoidable  in  any  in- 
breeding experiment.  In  maize,  the  difficulties  of  hand  pollina- 
tion result  in  the  selection  of  plants  whose  staminate  and  pistillate 
parts  are  matured  synchronously.  Any  great  differences  in  this 
respect,  particularly  towards  proterandry,  would  render  self- 
fertilization  difficult  or  impossible,  as  pollen,  according  to  Andro- 
nescu  ('15)  has  very  short  viability,  which  fact  my  own  experience 
confirms.  Of  course,  all  plants  which  are  weak,  sterile,  diseased 
or  in  any  way  abnormal  tend  to  become  eliminated  wherever 
these  causes  reduce  the  chance  of  obtaining  seed.  This  uncon- 
scious selection  becomes  more  rigid  as  reduction  in  vigor  and  pro- 
ductiveness increases  in  the  later  generations  of  inbreeding.  The 
small  amount  of  seed  produced  by  hand  pollination,  under  the 
most  favorable  circumstances  necessitates  the  using  of  the  best 


RESULTS    OF   INBREEDING.  43 

ears  obtained  for  planting  in  order  to  have  enough  plants  upon 
which  to  make  any  fair  observations. 

In  every  case  inbreeding  in  maize  has  so  far  resulted  in  a  reduc- 
tion in  size,  vigor  and  productiveness.  Some  thirty  or  forty 
inbred  strains  have  been  observed,  many  of  which  are  additional 
to  the  ones  reported  previously. 

From  the  preceding  statements  in  regard  to  the  effect  of  in- 
breeding it  can  be  said  that  this  process  produces  types  which 
differ  in  their  power  of  development  as  follows: 

1.  Plants  which  cannot  be  perpetuated. 

2.  Plants  which  fail  to  complete  normal  development  and  can 
be  propagated  only  with  the  greatest  difficulty. 

3.  Plants  which  are  perfectly  normal  but  varying  in  the  amount 
of  growth  they  attain  at  maturity. 

These  normal  inbred  plants,  so  far  obtained  in  maize,  are  not 
as  a  rule  as  large,  vigorous  or  productive  as  the  original  cross- 
fertilized  plants.  It  is  theoretically  possible  to  obtain  such  plants, 
which  cannot  be  reduced  in  vigor  in  a  homozygous  condition  as 
will  be  explained  later.  There  -is  some  evidence  from  the  experi- 
ments of  Darwin,  that  such  plants  have  been  obtained  by  in- 
breeding in  other  material,  for  example,  in  Ipomea  and  Mimulus. 
Selection  will  help  to  obtain  these  vigorous,  unreduceable  indi- 
viduals but  may  not  be  fully  effective  in  doing  so.  More  or  less 
unconscious  selection  is  unavoidable  in  any  inbreeding  experiment. 

These  homozygous,  normal,  inbred  strains,  after  the  reduction 
in  growth  has  ceased,  are  quite  comparable  to  plants  of  a  naturally 
self-fertilized  species.  Darwin  found  that  self-pollination  caused 
no  reduction  in  vigor  in  Nicotiana,  Pisum,  Lathyrus,  Phaseolus 
and  other  genera  which  are  naturally  self-fertilized  to  a  large 
extent.  Hayes  and  Jones  ('17)  have  found  similar  results  with 
the  tomato.  The  only  effect  that  inbreeding  may  have  on  such 
plants  is  merely  to  isolate  pure  lines,  which  are  quite  uniform 
among  themselves,  but  may  be  diverse  from  one  another,  as 
shown  by  soy  beans  (Jones  and  Hayes  ('17),  but  which  show  no 
reduction  in  vigor  on  continued  artificial  inbreeding.  These 
results  are  perfectly  in  accord  with  Johannsen's  genotype  con- 
ception. 


44         connecticut  expeeiment  station  bulletin  207. 

The  Approach  to  Complete  Homozygosity. 

It  now  remains  to  be  seen  whether  or  not  these  inbred  strains 
are  reaching  the  limit  of  reduction.  There  are  two  ways  of  de- 
termining this,  one  is  by  growing  two  successive  inbred  generations 
side  by  side  in  the  same  year,  the  other  is  by  crossing  different 
plants  within  the  same  inbred  strain. 

In  Table  9  the  results  from  two  successive  generations  grown 
side  by  side  in  the  same  year  are  compared.  On  the  whole,  an 
additional  year  of  inbreeding  after  the  sixth  produces  very  little 
change.  In  Table  10  are  given  the  height,  yield  and  length  of  ear 
of  selfed  and  sib-crossed  plants  which  were  grown  in  1917.  In 
1916,  in  each  of  the  strains  of  which  figures  are  given  in  the  table, 
some  plants  were  selfed  and  some  were  crossed  by  another  plant 
within  the  same  strain.  Since  all  the  plants  grown  that  year  in 
any  one  strain  came  from  one  individual  of  the  preceding  genera- 
tion, that  generation  is  the  significant  one.  In  other  words  if 
the  plant  in  that  generation  was  homozygous,  no  increase  of  the 
sib-crossed  plants  over  the  selfed  plants  would  be  expected.  The 
figures  show  that  there  is,  on  the  whole,  a  slight  increase  in  all 
the  characters  studied.  The  increase,  however,  is  no  greater  in 
the  cases  where  the  common  ancestor  was  inbred  for  seven  genera- 
tions than  in  the  cases  where  it  was  inbred  nine  generations. 

Shull  ('11)  compared  sib-crosses  with  selfed  plants  in  which 
the  significant  generation,  as  I  understand  it,  was  the  fourth,  and 
found  that  the  crossed  plants  slightly  excelled  the  selfed  plants  in 
height,  number  of  rows  on  the  ear  and  yield  of  grain.  Similarly 
the  Fi  X  Sibs  exceeded  Fi  X  self  in  yield,  showing  that  in  the* 
fourth  generation  complete  homozygosis  had  not  been  attained. 

Whether  or  not  complete  homozygosis  has  been  attained  by 
some  or  all  of  the  strains  shown  in  Table  10  cannot  be  stated 
positively  from  the  data  given.  In  most  cases  the  increase  of  the 
sib-crosses  over  the  selfs  is  slight  and  probably  of  no  significance 
as  there  are  about  an  equal  number  of  cases  in  which  the  reverse 
condition  is  shown.  A  few  of  the  sib-crosses  are,  however,  con- 
siderably greater  than  the  selfs  in  all  three  characters  and  it  may 
very  well  be  that  these  strains  have  not  attained  the  degree  of 
homozygosity  that  the  other  strains  have.  More  data  are  needed 
to  establish  this  point  with  certainty  as  environmental  factors 
which  favored  a  certain  plot  in  one  character  would  also  favor 
the  other  character  as  well. 


THE  APPROACH  TO  COMPLETE  HOMOZYGOSITY. 


45 


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heterozygosis  and  vegetative  luxuriance.  47 

The  Effect  of  Heterozygosis  on  Vegetative  Luxuriance. 

The  most  noticeable  manifestation  of  heterosis  in  plants  is  a 
general  increase  in  vegetative  luxuriance.  In  maize  this  is  par- 
ticularly noticeable  in  increased  height  of  plant,  diameter  of  stalk, 
root  development,  length  of  ear  and  productiveness  of  grain  (see 
Plates  III,  V,  VII,  VIII,  IX,  X  and  XII).  In  crosses  between 
inbred  strains  of  maize  the  amount  of  heterosis  shown  is  inversely 
proportional  to  the  degree  of  relationship  as  shown  in  Table  11, 
Montgomery  ('12)  has  obtained  similar  results. 

Some  characters  are  much  more  affected  by  heterozygosis  than 
others.  In  comparing  Tables  12,  13  and  14  with  Tables  15  and  16 
it  will  be  noticed  that  the  yield  of  the  crosses  is  increased  180 
per  cent.,  height  is  increased  27  per  cent,  and  length  of  ear  29 
per  cent,  over  the  average  of  their  parental  lines.  On  the  other 
hand,  the  number  of  nodes  per  plant  and  number  of  rows  of  grain 
on  the  ear  is  increased  only  6  and  5  per  cent,  respectively.  In 
other  words,  heterozygosis  does  not  increase  the  number  of  parts 
to  anything  like  the  extent  that  it  increases  the  size  of  those  parts. 
Those  parts  of  the  plants  which  are  more  or  less  indeterminate  in 
size,  like  internodes,  ears  and  seeds  are  augmented  by  crossing  as 
the  result  of  an  increase  in  the  rapidity  and  rate  of  cell  division. 
The  increase  in  size  of  parts  is  probably  brought  about  by  an 
increase  in  size  of  cells  as  well  as  an  enormous  increase  in  number 
of  cells.  Tupper  and  Bartlett  ('16)  have  shown  that  gigas  mutants 
in  Oenothera  have  larger  cells  than  the  non-mutant  type,  so  that 
a  change  in  cell  size  may  accompany  a  germinal  change. 

From  Table  11  it  will  also  be  seen  that  some  first  generation 
hybrids  may  even  surpass  the-  original  variety  in  yield,  height  or 
length  of  ear,  although  the  comparison  is  rather  unfair  as  the 
Learning  variety  was  not  acclimatized  as  were  the  inbred  strains. 
The  return  of  vigor  realized  in  the  first  generation  crosses  is  often 
enormous,  and  the  same  is  true  of  crossing  inbred  strains  derived 
from  totally  different  types  of  maize  as  is  shown  in  Table  17. 

Although  there  is  an  immediate  and  striking  return  to  the 
vigorous  condition  of  the  non-inbred  stock  there  is  not  a  return 
in  variability  as  shown  in  Tables  18,  19,  20,  21  and  22.  The  first 
generation  crosses  are  no  more  variable  than  the  inbred  strains 
by  which  they  are  produced,  in  many  cases  less  variable,  and  show 
striking  differences  when  compared  to  the  original  stock.  The 
coefficient    of  variability  is  entirely  inadequate  in  bringing  out 


48 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 


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HETEROZYGOSIS   AND    VEGETATIVE    LUXURIANCE. 


49 


Table  12.     The  effect  of  crossing  inbred  strains  of  maize 
as  shown  by  the  increase  in  the  yield  of  grain. 


Pedigree  number 
of  strain — A 

Yield  of  bushels  per 

acre 

Pedigree  number 
of  strain — B 

A 

AXB 

BX 

A                  B 

1-6-1-3-4-4-4-2-4-4 

34.9 

99.1 

99 

9                 30.3 

1-7-1-1-1-4-7-5-4-7 

1-6-1-3-4-4-4-2-4-4 

34.9 

112.9 

37.4 

1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 

34.9 

82.4 

30.7 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-4-1 

31.9 

101.0 

18.3 

1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 

16.8 

88.1 

84 

4                 30.3 

1-7-1-1-1-4-7-5-4-7 

1-6-1-3-4-4-4-2-5-5 

16.8 

106.2 

106 

7                 20.0 

1-7-1-2-2-9-2-1-1-4 

1-6-1-3-4-4-4-2-5-5 

16.8 

91.0 

30.7 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-5-3 

31.5 

94.8 

30.4 

1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 

30.5 
30.5 

63.9 
71.5 

31.5 
31.9 

1-6-1-3-4-4-4-2-5-3 
1-6-1-3-4-4-4-2-4-1 

1-9-1-2-4-6-7-5-3 

30.5 

58.0 

30.4 

1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 

30.5 

52.5 

100 

5                 18.9 

1-7-1-1-1-4-7-5-4-5 

1-9-1-2-4-6-7-5-3 

30.5 

59.6 

82 

1                 20.0 

1-7-1-2-2-9-2-1-1-4 

1-9-1-2-4-6-7-5-6 

30.7 

66.3 

18.3 

1-7-1-2-2-9-2-1-1-1 

1-7-1-2-2-9-2-1-1-4 
1-7^1-1-1-4-7-5-2-6 

20.0 

37.4 

84.9 
40.5 

34.9 
16.8 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

30.4 

59.4 

31.9 

1-6-1-3-4-4-4-2-4-1 

28.8 

78.4 

27.2 

50.4 

Percent  increase. .  .  . 

180.00 

Table  13.     The  effect  of  crossing  inbred  strains  of  maize 
AS  shown  by  the  increase  in  the  height  of  plant. 


Pedigree  number 
of  strain — A 

Height  of  plant  in  inches 

A 

AXB 

BXA 

B 

of  strain — B 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-4 

97. 8  ±.36 
97.8±.36 
97. 8  ±.36 
96. 7  ±.37 
93. 6 ±.57 
93 .  6  ± .  57 
93.  6  ±.57 
102.  7 ±.47 
80.3±.35 
S0.3±.35 
80.3±.35 
80. 3  ±.35 
80.3±.35 
77.0±.52 
82.6±.61 
78.5±.71 
82.2±.77 

117. 3±    .61 
117. 6±    .38 
115. 4±    .56 
121. 9±    .46 
112.9±1.04 
116. 1±    .41 
11_3.8±    .40 
116. 0±    .42 
111.1±    .61 
110. 5±    .58 
109. 2 ±    .76 
110. 9±    .60 
108. 1±    .50 
111.1±    .58 
114. 9±    .68 
98. 7±    .78 
105. 2±    .71 

117.2±.44 

90.2i.46 
78.5i.71 
77.0i.52 
91.2i.68 
90.2i.46 
82.'6±..61 
77.0i.52 
82.2i.77 
102.7i.47 
96.7i.37 
82.2i.77 
88.  7  i.  70 
82.6i.61 
91.2i.68 
97.  8  i.  36 

93 . 6  i . 57 

96. 7  i.  37 

1-7-1-1-1-4-7-5-4-7 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-4-1 

1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 

109. 9 ±    .77 
113. 4±    .51 

1-7-1-1-1-4-7-5-4-7 
1-7-1-2-2-9-2-1-1-4 
1-9- 1-2-4-6-7-5-61 J 
1-7-1-1-1-4-7-5-2-1 
1-6-1-3-4-4-4-2-5-3 

1-6-1-3-4-4-4-2-5-3 
1-9-1-2-4-6-7-5-3 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-4-1 

1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-6 

94. Oil. 36 
114. 1±    .55 
109. 5i    .76 

1-7-1-1-1-4-7-5-2-1 
1-7-1-1-1-4-7-5-4-5 
1-7-1-2-2-9-2-1-1-4 
1-7-1-2-2-9-2-1-1-1 

1-7-1-2-2-9-2-1-1-4 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

1-6-1-3-4-4-4-2-4-1 

Percent  increase. .  .  . 

88.0 

112.4 

24.2 
27.44 

88.3 

50 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 


Table  14.     The  effect  of  ceossing  inbred  strains  of  maize 
as- shown  by  the  increase  in  the  length  of  ear. 


Pedigree  number 

of  strain — A 

Length  of  ear  in  inches 

Pedigree  number 
of  strain — B 

A 

AXB 

BXA 

B 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-4 

6.1±.09 
6.1  ±.09 
6.1±.09 
6.2±.07 
6.0±.16 
6.0±.16 
6.0±.16 
6.9±.10 
5.9±.0o 
5. 9  ±.05 
5.9  ±.05 
5.9±.05 
5 . 9  ± .  05 
6 . 1 ± . 08 
5.1±.12 
4.3±.06 
4.2±.08 

7.1±.ll 
7.5±.ll 
7.8±.08 
7.9±.10 
7.5±.12 
8 . 2  ± . 08 
7.8±.0S 
7. 6  ±.09 
7.7±.09 
7 . 6  ± . 09 
6.5±.12 
6.5±.10 
7.1±.12 
7.1±.ll 
7. 6  ±.14 
5 . 5  ± . 12 
6.0±.10 

7.3±.10 

4 . 0  ± . 08 
4 . 3  ± . 06 
6 . 1 ± . 08 
5 . 3  ± . 09 
4.0±.08 
5.1±.12 
6 . 1 ± . 08 
4.2±.08 
6.9±.10 
6.2±.07 
4 . 2  ± . 08 

4 . 2  ± .  09 
5.1±.12 

5 . 3  ± .  09 
6.1±.09 
C.0±.16 
6.2±.07 

1-7-1-1-1-4-7-5-4-7 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-4-1 

1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 

6.8±.15 
8.0±.09 

1-7-1-1-1-4-7-5-4-7 
1-7-1-2-2-9-2-1-1-4 
1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-5-3 

1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-5-3 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-4-1 

1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-6 

5.5±.ll 
7.6±.09 
7. 6  ±.13 

1-7-1-1-1-4-7-5-2-1 
1-7-1-1-1-4-7-5-4-5 
1-7-1-2-2-9-2-1-1-4 
1-7-1-2-2-9-2-1-1-1 

1-7-1-2-2-9-2-1-1-4 

1-6-1-3-4-4-4-2-4-4 

1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

1-6-1-3-4-4-4-2-4-1 

5.8 

7.2 

1.6 

28.57 

5.3 

Percent  increase. . .  .. 

Table  15.     The  effect  of  crossing  inbred  strains  of  maize 
as  shown  by  the  increase  in  the  number  of  nodes. 


Number  of  nodes 

of  strain — A 

A 

AXB 

BXA 

B 

of  strain — B 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-4 

12. 7  ±.06 
12.7±.06 
12.7±.06 
11.5±.05 
12.4±.08 
12.4±.0S 
12. 4  ±.08 
12.2±.06 
13.1 ±.06 
13.1  ±.06 
13.1  ±.06 
13.1  ±.08 
13.1  ±.06 
12.9±.07 
12.1  ±.09 
ll.8i.07 
11.6±.10 

13. 6  ±.07 

13. 3  ±.04 
14.0  ±.05 
14.0±.05 
12.9±.08 

13. 5  ±.06 

13. 4  ±.06 
13. 3  ±.06 
12.8±.03 
12.9±.06 
13.  2  ±.03 
13. 3  ±.06 
13. 3  ±.07 
13.  7  ±.08 

13. 6  ±.06 
11.3±.07 
12.6±.07 

13. 2  ±.07 

12. 2  ±.07 
11.S±.07 
12.9±.07 
13.0±.0S 
12. 2  ±.07 
12.1  ±.09 
12.9±.07 
11.6±.10 
12.2±.06 
11. 5  ±.05 
11.6±.10 
12..  7  ±.10 
12.1  ±.09 
13.0±.08 
12.7±.06 

12. 4  ±.08 

11. 5  ±.05 

1-7-1-1-1-4-7-5-4-7 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-4-1 

1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 

13. 2  ±.05 
13.1  ±.06 

1-7-1-1-1-4-7-5-4-7 
1-7-1-2-2-9-2-1-1-4 
1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-5-3 

1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-5-3 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-4-3 

1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-6 

12.5±.13 
14.0  ±.06 
13. 7  ±.07 

1-7-1-1-1-4-7-5-2-1 
1-7-1-1-1-4-7-5-4-5 
1-7-1-2-2-9-2-1-1-4 
1-7-1-2-2-9-2-1-1-1 

1-7-1-2-2-9-2-1-1-4 

1-6-1-3-4-4-4-2-4-4 

1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

1-6-1-3-4-4-4-2-4-1 

Increase 

Percent  increase.  .  .  . 

12.5 

13.2 

.S 
6.45 

12.3 

HETEROZYGOSIS   AND    VEGETATIVE    LUXURIANCE. 


51 


Table  16.     The  effect  of  crossing  inbred  strains  of  maize  as  shown 
by  the  increase  in  the  number  of  rows  of  grain  on  the  ear. 


Pedigree  number 
of  strain — A 

Number  of  rows  of  grain  on  the  ear 

Pedigree  number 
of  strain — B 

A 

AXB 

BXA 

B 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-4 

16. 9  ±.14 
16. 9  ±.14 
16.9±.14 
15. 7  ±.09 
14.1±.10 
14.1±.10 
14.1  ±.10 

14. 3  ±.10 
15.4±,08 

15. 4  ±.08 
15. 4  ±.08 
15. 4  ±.08 
15. 4  ±.08 
15.5±.13 
15.9±.15 
21.8±.16 
22.0±.20 

19. 5  ±.15 
19. 5  ±.13 
17, 2  ±'.13 

18.  4  ±.13 
17.4±.ll 
16.9±.10 
17.0 ±.10 
19. 4  ±.15 
15.7±.ll 
16. 7  ±.12 
19. 9  ±.20 
17.8±.14 
16.8±.17 
16.  2 ±.14 
19.3±.17 
17.6±.14 

19.  8  ±.16 

20.8±.15 

21.8±.15 
21.8±.16 
15. 5  ±.13 
15.9±.15 
21.8±.15 
15.9±.15 
15.  5 ±.13 
22.0±.20 
14.3±,10 
15. 7  ±.09 
22.0 ±.20 
20.1  ±.20 
15.9±.15 
15.9±.15 
16.9±.14 
14.1  ±.10 
15.  7  ±.09 

1-7-1-1-1-4-7-5-4-7 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-4-1 

1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 

18. 3  ±.14 
18. 2  ±.13 

1-7-1-1-1-4-7-5-4-7 
1-7-1-2-2-9-2-1-1-4 
1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-5-3 

1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-5-3 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-4-1 

1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-6 

18. 7  ±.25 
19.0±.17 
16.  2 ±.09 

1-7-1-1-1-4-7-5-2-1 
1-7-1-1-1-4-7-5-4-5 
1-7-1-2-2-9-2-1-1-4 
1-7-1-2-2-9-2-1-1-1 

1-7-1-2-2-9-2-1-1-4 

1-6-1-3-4-4-4-2-4-4 

1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

1-6-1-3-4-4-4-2-4-1 

16.2 

17.9 
.9 
5.29 

17.7 

Increase 

Percent  increase. .  .  . 

Table  17.     The  effect  of     crossing  inbred  strains  derived 
from  different  types  of  maize. 


Yield 

Increase 

Increase 

Length 

Increase 

Type 

Pedigree  number 

bu. 

above 

Height 

above 

of  ear 

above 

per 

ave.  of 

inches 

ave.  of 

inches 

ave.  of 

acre 

parents 

parents 

parents 

Dent 

1-6-1-3-4-4-4-2-4-4-3 

34.9 

97.8 

6.1 

Floury  

10-3-7-3-9-7-5-4-3 

10.4 

75.5 

6.1 

Flint 

29-5-2-3-8 : 

9.2 
28  4 

88.7 
54  3 

5.9 
5  6 

Pop 

65-8-2-2-6-5  2  4 

Dent  X  Floury. . 

(1-6-1-3)  X  (10-3-7-3).  . 

90.4 

+67.7 

122.8 

+36.1 

9.1 

+3.0 

Dent  X  Flint .  . . 

(1-6-1-3)  X  (29-5-2-3).. 

94.6 

+72.5 

117.5 

+24.2 

8.9 

+2.9J 

Floury  X  Dent.  . 

(10-3-7-3) X (1-6-1-3).. 

43.3 

+20.6 

108.2 

+21.5 

7.4 

+  1.3 

Floury  X  Flint. . 

(10-3-7-3)  X  (29-11-4-4) 

61.1 

+  51.3 

104.5 

+22.4 

9.7 

+3.7 

Flint  X  Dent .  .  . 

(29-5-2-3) X (1-6-1-3).  . 

80.7 

+58.6 

115.7 

+22.4 

9.6 

+3.6 

Flint X  Floury. . 

(29-5-2-3) X (10-3-7-3). 

73.0 

+63.2 

112.9 

+30.8 

10.0 

+4.0 

Pop  X  Dent .... 

(65-8-2-2) X (1-6-1-3).. 

73.1 

+41.4 

88.9 

+  12.8 

7.2 

+  1.3 

Pop X Flint.  ..  . 

(65-8-2-2) X (5-8-6-3) . . 

51.3 

79.5 

7.1 

52 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 


Table  18.     The  effect  of  crossing  upon  variability  as  shown 
by  the  height  of  plant. 


Pedigree  number 
of  strain — A 

Coefficient  of  variability  of  height 

Pedigree  number 
of  strain — B 

A 

AXB 

BXA 

B 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-4 

4 .  14  ± .  26 
4. 14  ±.26 
4. 14  ±.26 
4. 40  ±.27 
6. 14  ±.43 
6. 14  ±.43 
6. 14  ±.43 
5.11±.32 
5. 04  ±.31 

5. 04  ±.31 
5.04±.31 
5.Q4±.31 
5.04±.31 
7.73±.48 

8 . 05  ± . 52 
9 . 75  ± . 64 

10. 64  ±.67 

6.10±.37 
3 .  74  ± .  23 
5 . 55  ± . 34 
4. 22 ±.27 
9.92±.66 
4 . 09  ± . 25 
4 . 00  ± . 25 
4 . 05  ± . 26 
6. 66  ±.39 
6. 15  ±.38 
7.78±.49 
6.18±.38 
5 . 27  ± . 33 
6. 26 ±.38 
6.27±.41 
8.81±.56 
7.41±.47 

4. 01  ±.26 

5. 49  ±.36 
9 . 7.5  ± .  64 
7.73±.48 
7.95±.52 
5.49±.36 
8. 05 ±.52 
7. 73  ±.48 

10. 64  ±.67 
5.11±.32 
4. 40  ±.27 

10. 64  ±.67 
7.27±.56 
8. 05 ±.52 
7. 95  ±.52 
4. 14  ±.26 
6. 14  ±.43 
4. 40  ±.27 

1-7-1-1-1-4-7-5-4-7 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-4-1 

1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 

7. 32  ±.50 
5.20±.32 

1-7-1-1-1-4-7-5-4-7 
1-7-1-2-2-9-2-1-1-4 
1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-5-3 

1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-5-3 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-4-1 

1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3 
1-9-1-2-4-6-7-5-6 

12.61±1.01 
5. 48 ±.34 
7.94±.49 

1-7-1-1-1-4-7-5-2-1 
1-7-1-1-1-4-7-5-4-5 
1-7-1-2-2-9-2-1-1-4 
1-7-1-2-2-9-2-1-1-1 

1-7-1-2-2-9-2-1-1-4 

1-6-1-3-4-4-4-2-4-4 

1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

1-6-1-3-4-4-4-2-4-1 

5.98 

6.03 

7.11 

Table  19.     The  effect  of  crossing  upon  variability  as  shown 
by  the  length  of  ear. 


Coefficient  of  variability  of  length  of  ear 

of  strain — A 

A 

AXB 

BXA 

B 

of  strain — B 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-4 

17.17±1.18 

17.17±1.1S 

17, 17.±1. 18 

13.06±    .82 

26.17±1.95 

26.17±1.95 

26.17±1.95 

16. 23  ±1.05 

9.32±    .58 

9.32±    .58 

9.32±    .58 

9.32±    .58 

9.32±    .58 

13.77±    .90 

23. 33  ±1.77 

15.58±    .96 

20.00±1.31 

19.86±1.1S 
17.33±1.02 
11.92±    .74 
14.94±    .93 
17.73±1.11 
11.71±    .72 
12.44±    .76 
13.55±    .85 
14.16±    .87 
13.82±    .84 
20.46±1.36 
17.69±1.15 
18.87±1.23 
18.73±1.16 
19.  87  ±1.30 
24.91±1.66 
18.83±1.21 

16.99±1.05 

22.75±1.51 
15.58±    .96 
13.77±    .90 
16.04±1.20 
22.75±1.51 
23. 33  ±1.77 
13.77±    .90 
20.00±1.31 
16.  23  ±1.05 
13.06±    .82 
20.00±1.31 
19. 76  ±1.62 
23.33±1.77 
16. 04  ±1.20 
17.17±1.18 
26.17±1.95 
13.06±    .82 

1-7-1-1-1-4-7-5-4-7 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-1 

1-9-1-2-4-6-7-5-6 
1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-3 

25.88±1.65 
12.37±    .77 

1-7-1-1-1-4-7-5-4-7 
1-7-1-2-2-9-2-1-1-4 
1-9-1-2-4-6-7-5-6 
1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 

1-9-1-2-4-6-7-5-3 

1-9-1-2-4-6-7-5-3 

1-9-1-2-4-6-7-5-3 

1-9-1-2-4-6-7-5-3 

1-9-1-2-4-6-7-5-6 

1-7-1-2-2-9-2-1-1-4 

1-7-1-1-1-4-7-5-2-6 

15.45±1.47 
13.55±    .83 
20.26±1.26 

1-6-1-3^4-4-4-2-5-3 
1-6-1-3-4-4-4-2-4-1 
1-7-1-1-1-4-7-5-2-1 
1-7-1-1-1-4-7-5-4-5 
1-7-1-2-2-9-2-1-1-4 
1-7-1-2-2-9-2-1-1-1 
1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

1-6-1-3-4-4-4-2-4-1 

16.39 

16.87 

18.40 

HETEROZYGOSIS   AND    VEGETATIVE    LUXURIANCE. 


53 


Table  20.     The  effect  of  crossing  upon  variability  as  shown 
by  the  number  of  nodes. 


Pedigree  number 
of  strain — A 


Coefficient  of  variability  of  number  of  nodes 


A 


AXB 


BXA. 


B 


Pedigree  number 
of  strain — B 


1-6-1- 
1-6-1- 
1-6-1- 
1-6-1- 
1-6-1- 
1-6-1- 
1-6-1- 
1-6-1- 
1-9-1- 
1-9-1- 
1-9-1- 
1-9-1- 
1-9-1- 
1-9-1- 
1-7-1- 
1-7-1- 
1-7-1- 


3-4-4-4- 
3-4-4-4- 
3-4-4-4- 
3-4-4-4- 
3-4-4-4- 
3-4-4-4- 
3-4-4-4- 
3-4-4-4- 
2-4-6-7- 
2-4-6-7- 
2-4-6-7- 
2-4-6-7- 
2-4-6-7- 
2-4-6-7- 
2-2-9-2- 
1-1-4-7- 
1-1-4-7- 


2-4-4 

2-4-4 

2-4-4 

2-4-1 

2-5-5 

2-5-5 

2-5-5 

2-5-3 

5-3 

5-3 

5-3 

5-3 

5-3 

5-6 

1-1-4 

5-2-6 

5-2-1 


5.12±.32 
5.12±.32 
5.12±.32 
4. 61  ±.28 
6.94 ±.47 
6. 94  ±.47 
6. 94  ±.47 
5.98±.37 
5.11±.31 
5  ."'11  ±.31 
5.11±.31 
5.11±.31 
5.11±.31 
6.67±.41 
7.93±.50 
6. 27  ±.40 
9. 66  ±.60 


5.88±.36 
3. 68 ±.22 
4. 50 ±.27 
4.00±.25 
6 . 82  ± . 43 
5. 48  ±.32 
5.  67 ±.35 
5. 19  ±.32 
5. 47  ±.33 
5.  66 ±.35 
5.53±.34 
5.41 ±.34 
5. 86  ±.36 
5. 62  ±.34 
5. 15  ±.33 
7.61±.47 
6.67±.42 


5.91±.37 


4 . 09  ± . 27 
5 . 34  ± . 33 


8. 72 ±.73 
4.93±.30 
6.20±.38 


6 .  23  ± .  39 
6.  27 ±.40 
6.67±.41 
6.77±.45 
6 .  23  ± . 39 
7. 93  ±.50 
6.67±.41 
9. 66 ±.60 
5.98±.37 
4.61±.28 
9. 66  ±.60 
7.48±.56 
7.93±.50 
6 . 77  ± . 45 
5.12±.32 
6.94 ±.47 
4.61±.28 


1-7-1- 
1-7-1- 
1-9-1- 
1-7-1- 
1-7-1- 
1-7-1- 
1-9-1- 
1-7-1- 
1-6-1- 
1-6-1- 
1-7-1 
1-7-1- 
1-7-1- 
1-7-1- 
1-6-1- 
1-6-1- 
1-6-1- 


1-1-4- 
1-1-4- 
2-4-6- 
2-2-9- 
1-1-4- 
2-2-9- 
2-4-6 
■1-1-4 
3-4-4 
3-4-4 
1-1-4 
1-1-4 
2-2-9 
2-2-9 
3-4-4- 
3-4-4- 
3-4-4- 


7-5-4-7 

7-5-2-6 

7-5-6 

■2-1-1-1 

■7-5-4-7 

•2-1-1-4 

•7-5-6 

■7-5-2-1 

■4-2-5-3 

■4-2-4-1 

■7-5-2-1 

■7-5-4-5 

■2-1-1-4 

•2-1-1-1 

■4-2-4-4 

■4-2-5-5 

•4-2-4-1 


Average. 


6.05 


5.54 


Table  21.     The  effect  of  crossing  upon  variability  as  shown 
by  the  number  of  rows  of  grain  on  the  ear. 


Pedigree  number 
of  strain — A 

Coefficient  of  variability  of  number  of  rows 

A 

AXB 

BXA 

B 

of  strain — B 

1-6-1-3-4-4-4-2-4-4 
1-6-1-3-4-4-4-2-4-4 

9. 83  ±.61 
9.83±.61 
9. 83  ±.61 
6. 48  ±.40 
7.23±.51 
7.23±.51 
7.23±.51 
7.96±.50 
6 . OS  ± . 38 
6 .  08  ± .  38 
6.08±.38 
6 . 08  ± . 38 
6 . 08  ± . 38 
9.  21  ±.59 
9.40±.68 
8.43±.51 
10.  10  ±.64 

9.  73  ±.55 
8. 50  ±.48 
8.79±,54 
7. 96  ±.49 
7.31±.45 
6. 62 ±.40 
6.  60  ±.40 
8. 92  ±.55 
8. 00 ±.48 
8.36±.50 
11.11±.72 
8. 92 ±.56 
11. 35 ±.72 
10.42±.63 
9. 68  ±.63 
8.75±.56 
8. 84  ±.56 

8.  68 ±.52 

7. 48  ±.49 
8.43±.51 
9. 21  ±.59 
9. 06  ±.66 
7.48±.49 
9. 40  ±.68 
9. 21  ±.59 

10. 10  ±.64 
7. 96  ±.50 
6. 48  ±.40 

10. 10  ±.64 
8.94±.71 
9. 40 ±.68 
9.  06 ±.66 
9.83±.61 
7. 23  ±.51 
6.48±.40 

1-7-1-1-1-4-7-5-4-7 
1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-4-4 

1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-4-1 

1-7-1-2-2-9-2-1-1-1 

1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 
1-6-1-3-4-4-4-2-5-5 

9 . 24  ± . 55 
_8.11±.49 

1-7-1-1-1-4-7-5-4-7 
1-7-1-2-2-9-2-1-1-4 
1-9-1-2-4-6-7-5-6 

1-6-1-3-4-4-4-2-5-3 

1-7-1-1-1-4-7-5-2-1 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-5-3 

1-9-1-2-4-6-7-5-3 

1-6-1-3-4-4-4-2-4-1 

1-9-1-2-4-6-7-5-3 
1-9- ] -2-4-6-7-5-3 
1-9-1-2-4-6-7-5-3   . 
1-9-1-2-4-6-7-5-6 

10.32±.98 
10.62±.65 

7.02±.42 

1-7-1-1-1-4-7-5-2-1 
1-7-1-1-1-4-7-5-4-5 
1-7-1-2-2-9-2-1-1-4 
1-7-1-2-2-9-2-1-1-1 

1-7-1-2-2-9-2-1-1-4 

1-6-1-3-4-4-4-2-4-4 

1-7-1-1-1-4-7-5-2-6 

1-6-1-3-4-4-4-2-5-5 

1-7-1-1-1-4-7-5-2-1 

1-6-1-3-4-4-4-2-4-1 

Average 

7.83 

8.82 

8.58 

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HETEROZYGOSIS   AND    VEGETATIVE    LUXURIANCE.  55 

the  beautiful  uniformity  of  these  crosses  between  inbred  strains. 
In  every  respect  each  plant  is  a  replica  of  the  other.  A  collection 
of  such  vigorous  and  uniform  maize  plants  in  the  field  is  a  novel 
sight  (see  Plates  IHb  and  Vb). 

Shull  ('14)  has  pointed  out  that  vigorous  plants  may  be  less 
susceptible  to  the  effect  of  the  environment  than  weaker  types 
and  that  first  generation  hybrids,  between  uniform  strains,  may 
even  show  a  reduction  in  variability. 

The  results  obtained  show  this  quite  noticeably.  Particularly 
was  this  true  of  several  FVs  grown  between  their  parental  strains 
in  a  demonstration  plot  on  rich  low  ground.  During  both  seasons 
(1916— '17)  when  they  were  grown  on  this  piece  of  ground,  the 
weather  was  especially  unfavorable  when  the  plants  were  just 
starting,  the  ground  being  saturated  with  water  most  of  the  time. 
The  germination  in  the  selfed  lines  was  extremely  poor  and  many 
plants  which  did  grow  were  stunted,  and  remained  so  throughout 
the  season  and  never  attained  full  height  nor  did  they  produce 
either  tassels  or  ears.  The  variability  of  height,  in  these  plants, 
was  far  greater  than  in  many  non-inbred  varieties.  Several 
plants,  when  killed  by  frost  in  the  fall,  were  not  over  30  inches  tall 
while  the  average  height  of  this  strain  is  from  80  to  85  inches. 
The  hybrids  also  had  a  poorer  start  than  non-inbred  varieties 
grown  on  the  same  ground  on  account  of  the  small  seed,  but  were 
able  to  overcome  their  handicap  and  in  a  few  weeks  were  quite 
uniform.  At  the  end  of  the  season  the  difference  in  variability 
between  the  Fi  on  the  one  hand  and  the  inbred  strains  and  the 
varieties  on  the  other  was  striking.  These  plants  were  not  used 
in  the  statistical  work  given  here.  The  crosses  and  parents  which 
were  used  and  which  were  apparently  quite  uniform  show  a  slight 
reduction  in  variability,  in  the  number  of  nodes  and  in  height  in 
the  Fi's  as  compared  with  their  parents  as  can  be  seen  in  Tables 
18  and  20.  As  Shull  also  pointed  out,  the  variability  of  some 
characters  may  be  increased  by  heterosis.  This  is  shown  in  number 
of  rows  on  the  ear.  The  inbred  strains  rarely  or  never  produce  a 
second  ear.  The  vigorous  hybrids  almost  always  do,  and  as  the 
data  have  been  obtained  by  counting  all  the  ears  gathered  from  a 
plot,  the  variability  of  the  crosses,  as  shown  in  Tables  19  and  21, 
consequently  seems  greater  than  it  really  is  as  the  second  ear  on 
nearly  every  plant  is  smaller  and  contains  a  fewer  number  of  rows. 


56 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 


Although  reciprocal  crosses  are  on  the  whole  nearly  equal  in 
respect  to  the  degree  in  which  heterosis  is  shown,  there  is  some  evi- 
dence, from  Table  12,  that  this  is  not  always  so.  Observations 
from  the  crosses  in  the  field  showed  clearly  that  those  in  which 
strain  Number  1-6  was  used  as  the  female,  were  usually  more 
vigorous  and  productive  than  the  others.  In  Table  23  the  yields 
of  all  the  crosses  and  reciprocal  crosses  (from  1  to  4  of  each)  having 
the  same  parental  races  are  averaged.     An  average  of  all  those 


Table  23.     Yield  of  reciprocal  crosses  among  inbred  strains  of 

MAIZE. 
(All  crosses  grown  1916.     Yield  given  as  bushels  per  acre.) 


1-9-1-2    d1 


1-7-1-2    o" 


1-7-1-1    <? 


1-6-1-3    cf 


Average  9 

Yield  selfed: 

(1917) 
(1916) 

Ave.  weight 
of  seed-cg. 


1-9-1-2 

9 

1-7-1-2 

9 

1-7-1-1 

9 

1-6-1-3 

9 

82.1 

100.5 

86.7 

63.0 

70.9 

103.6 

55.3 

57.2 

98.7 

67,7 

95.8 

92.2 

62.0 


31.8 
30.6 


16.6 


7S.4 


37.6 
19.2 


27.9 


87.9 


42.3 

32.7 


19.9 


96.3 


46.2 
32.8 


34.1 


Average  o" 
89.8 

79.2 

70.4 

85.2 


crosses  in  which  each  strain  was  used  as  the  male  and  in  which 
each  was  used  as  the  female  parent  shows  some  striking  results. 
Those  crosses  on  the  whole  in  which  strain  Number  1-9  was  used 
as  the  female  gave  the  lowest  yield.  Those  crosses  in  which  strain 
Number  1-6  was  used  as  the  female  are  clearly  the  most  pro- 
ductive. Strain  Number  1-6  is  the  one  which  has  the  largest 
seeds  and  in  which  the  pistillate  inflorescence  is  the  best  developed 
of  the  four  strains  and  at  the  expense  of  the  staminate  inflorescence. 


HETEROZYGOSIS   AND    VEGETATIVE    LUXURIANCE.  57 

Strain  Number  1-9  is  just  the  reverse  of  this.  It  is  the  best  de- 
veloped of  all  the  inbred  strains  in  its  staminate  inflorescence, 
always  producing  abundant  pollen,  but  has  the  smallest  seeds, 
and  is  one  of  the  poorest  in  the  development  of  its  pistillate  in- 
florescence. Approximately  a  uniform  stand  of  plants  was  ob- 
tained in  all  these  crossed  plots.  They  were  all  grown  side  by  side 
in  the  same  field  in  the  same  year.  *  There  seems,  therefore,  to  be 
a  marked  correlation  in  the  development  of  the  pistillate  infloresc- 
ence between  the  mother  and  her  hybrid  progeny.  The  high  yield 
of  the  crosses  in  which  Number  1-9  was  used  as  the  male  is  due 
to  the  fact  that  its  average  yield  was  not  pulled  down  by  the  low 
yielding  crosses  in  which  it  was  used  as  a  female.  The  crosses  in 
which  1-7-1-1  and  1-7-1-2  were  used  cannot  be  compared  fairly 
with  the  other  two  because  these  two  strains  are  more  closely 
related.  This  correlation  bears  a  close  relationship  with  the  size 
and  development  of  the  seed  which  produces  first  generation 
hybrid  plant.  The  seeds  of  strain  1-9  are  the  poorest  developed, 
those  of  Number  1-6  are  the  best.  Hence,  the  plants  of  crosses 
(1-6)  x  (1-9)  have  a  better  start  than  the  plants  of  the  reciprocal 
cross.  This  assumption  is  borne  out  by  the  fact  that  the  second 
generation  starting  from  large  fully  developed  seeds  grown  on 
vigorous  Fi  plants  are  larger  at  the  start  than  the  Fi  plants  grown 
from  small,  poorly  developed  seeds  produced  on  inbred  plants. 
This  is  shown  in  Fig.  Ill  and  Plate  IX.  The  second  generation, 
however,  is  surpassed  by  the  first  before  the  end  of  the  season,  as 
shown  in  Fig.  Ill  and  Plate  X.  Somewhat  similar  results  have 
been  obtained  by  Castle  ('16)  in  guinea-pigs.  F2  animals,  out  of 
vigorous  Fi  females,  are  larger  at  the  start  than  either  parent 
but  do  not  surpass  the  Fi  individuals  as  in  this  case.  It  will  be 
seen  from  this  that  in  plants  or  animals  which  are  reduced  by 
inbreeding,  the  Fi  is  handicapped  in  comparison  with  the  F2  and 
the  immediately  following  generations. 

It  is  not  certain  that  the  differences  between  reciprocal  crosses 
can  be  accounted  for  on  a  purely  nutritional  basis.  There  is  the 
possibility  of  unequal  germinal  reactions  with  different  cytoplasms. 


*The  crossed  strains  were  not  grown  between  their  inbred  parental  strains 
as  was  the  case  in  the  yields  reported  in  U.  S.  Dept.  of  Agric,  B.  P.  I. 
Bull.  243.  This  accounts  in  part  for  the  extraordinarily  large  yields 
obtained  at  that  time. 


58 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 


100 


75 


1 


50 


25- 


Growth  Curves  of 
Two  Inbred  Strains 
of  Maize  and  Their 
Fi  and  F«  Hybrids. 


(1-7) 


30      40      50      60      70      80 

Number  of  Days  from  Planting 


96 


100 


Figure^III.     Growth    curves    of  two  inbred  strains  of  maize  and  their 
first  and  second  generation  hybrids. 


inbreeding  in  plant  and  animal  improvement.        59 

The  Value  of  Inbreeding  in  Plant  and  Animal  Improvement. 

These  inbreeding  and  crossbreeding  experiments  on  corn  have 
considerable  theoretical  importance  in  the  improvement  of  culti- 
vated plants  and  domesticated  animals.  We  have  seen  that  in- 
breeding results  in  the  elimination  of  abnormal,  pathological  and 
undesirable  characters  in  general.  This  result  has  been  obtained 
with  a  loss  of  size,  vigor  and  productiveness.  When  these  inbred 
strains  are  crossed,  however,  vigor  and  productiveness  are  re- 
turned in  increased  amount  due  to  the  uniform  excellence  of  the 
individuals  freed  from  undesirable  characters.  In  this  way  a  new 
variety  or  breed  can  be  synthesized  from  the  purified  inbred 
strains  of  an  old  stock.  A  great  sacrifice  is  thus  made  to  attain  a 
great  good.  Of  course  such  a  variety  would  have  to  be  fixed  by 
selection  during  a  number  of  generations.  The  common  practice 
of  crossing  in  animals  and  plants  already  extremely  heterozygous 
in  order  to  obtain  further  improvement  is  like  trying  to  solve  a 
picture  puzzle  in  the  dark.  It  is  only  by  resolving  a  naturally 
crossed  species  into  homozygous  types  by  inbreeding  that  it  can 
be  best  analyzed  and  its  desirable  characters  most  surely  selected 
for  the  recreation  of  an  improved  type. 

The  practical  value  of  inbreeding  has  long  been  recognized  by 
the  breeders  of  domesticated  animals.  To  gain  uniformity  and 
the  highest  expression  of  certain  desirable  characters  they  often 
practice  inbreeding  until  the  vigor  of  the  breed  is  frequently  im- 
paired. From  the  results  obtained  with  maize  it  seems  that  they 
stop  just  before  the  greatest  good  is  to  be  accomplished.  What  if 
vigor  is  lost?  It  can  always  be  regained  immediately  by  crossing. 
There  is  no  surer  way  of  eliminating  undesirable  characters  and  dis- 
covering the  best  that  there  is  in  a  stock  than  by  a  process  of  rigid 
inbreeding  followed  by  subsequent  testing  in  different  crosses.  This 
is  not  offered  as  a  practical  plan  of  procedure  for  the  improvement 
of  animals.  It  is  merely  intended  to  call  attention  to  a  principle 
which  has  probably  not  been  used  to  its  fullest  extent.  It  may  be 
that  many  domesticated  breeds  of  animals  cannot  endure  in- 
breeding to  the  extent  that  maize  can.  The  cost  of  obtaining  such 
pure  types  might  very  easily  be  prohibitive.  The  writer  believes^ 
however,  that  the  splitting  up  of  a  breed  of  animals  or  a  naturally 
crossed  variety  of  plants  by  long  continued  inbreeding  of  the 
closest  kind  possible  followed  by  the  recombination  of  the  most 
desirable  inbred  types,  obtained  in  sufficient  numbers  to  insure 


60  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

that  nothing  of  value  present  in  the  stock  at  the  start,  is  lost,  is  a 
valuable,  practicable  method  of  improvement  in  many  animals  and 
plants.  According  to  this  method  a  variety  or  breed  would  be  re- 
created and  then  continued  in  a  naturally  crossed  condition  just  as 
it  was  before.  The  value  of  this  procedure  as  a  method  for  plant 
improvement  is  now  being  tested.  It  is,  of  course,  a  long  time 
proposition  and  one  that  must  be  carried  on  extensively  to  promise 
results. 

With  a  few  plants  which  are  easily  crossed  it  is  possible  to 
utilize  hybrid  vigor  to  the  fullest  extent  by  growing  only  first 
generation  plants.  Attention  has  been  repeatedly  called  to  this 
method  of  increasing  the  productiveness,  particularly  of  maize 
and  tomatoes.  The  greatest  amount  of  hybrid  vigor  is  shown  in 
maize  when  the  plants  have  been  previously  inbred.  Unfortu- 
nately, when  the  inbreeding  is  carried  on  for  several  generations 
the  reduction  in  the  vigor  of  the  resulting  plants  is  so  great  that 
the  small  size  and  low  vitality  of  the  seeds  borne  on  inbred  plants 
seriously  handicaps  the  hybrid  plants  grown  from  these  seeds  as 
just  shown.  So  what  is  gained  by  an  increased  amount  of  heterosis 
may  be  partly  lost  by  the  poor  start  which  the  plants  have.  This 
handicap,  in  comparison  with  normally-crossed  varieties,  the  Fi 
may  not  be  able  to  overcome  entirely  even  though  it  is  far  more 
uniform  and  free  from  barren,  mal-formed  and  otherwise  unde- 
sirable plants — factors  which  count  heavily  in  maximum  pro- 
duction. 

A  way  to  overcome  this  handicap  suggests  itself  which  is  to 
cross  two  vigorous  first  generation  hybrids  whose  composition  is 
such  that  the  resulting  cross  will  not  be  less  heterozygous  than 
either  parent  and,  therefore,  theoretically  no  less  vigorous  and 
productive.  This  is  easily  accomplished  by  taking  four  distinct 
inbred  strains  which  are  of  such  a  composition  that  a  cross  between 
any  two  of  them  gives  a  vigorous  product.  Now  by  crossing  two 
of  these  strains  to  make  one  first  generation  hybrid,  and  at  the 
same  time  crossing  the  other  two  to  make  another,  and  then  by 
combining  the  two  first  generation  hybrids  there  should  be  no 
reduction  in  heterozygosity.  These  doubly  crossed  plants,  how- 
ever, starting  from  large  seeds  produced  on  large,  vigorous  hybrid 
plants  would  be  freed  from  the  handicap  which  their  parents  had 
and  although  somewhat  less  uniform  should  be  more  productive. 
While  it  may  be  out  of  place  to  say  anything  about  this  method 


HETEROZYGOSIS    AND    SELECTIVE    FERTILIZATION.  61 

until  it  has  been  thoroughly  tested  it  is  a  method  which  is  more 
promising  than  the  plan  originally  advocated  because "  by  this 
method  crossed  seed  for  general  field  planting  is  produced  much 
more  abundantly  than  when  non-vigorous  inbred  strains  are 
crossed. 


The  Effect  of  Heterozygosis  upon  Endosperm  Develop- 
ment and  Selective  Fertilization. 

Together  with  the  increase  in  size  of  other  parts  of  the  plant 
there  is  also  an  appreciable  increase  in  the  size  and  weight  of  seeds 
of  maize  immediately  resulting  from  cross-pollination.  This  has 
been  shown  clearly  by  Collins  and  Kempton  ('13)  by  pollinating 
several  ears  of  maize  with  a  mixture  of  the  plant's  own  pollen  and 
that  of  a  different  variety.  Roberts  ('12),  Carrier  ('13)  and  Wolfe 
(*15)  have  also  shown  that  in  maize  the  endosperm  is  increased  by 
crossing.  The  writer  ('18)  has  shown  that  this  increase  in  endo- 
sperm development  appears  even  more  strikingly  in  reciprocal 
crosses  between  different  inbred  strains  of  maize.  At  that  time 
reciprocal  crosses  had  not  been  obtain'ed  between  different  indi- 
vidual plants.  In  Table  24  are  given  the  distributions  of  the 
weights  of  the  seeds  shown  in  Plate  XIa.  Two  plants  were  pol- 
linated with  a  mixture  of  pollen  obtained  from  these  same  two 
plants.  One  of  the  plants  had  white  seeds  and  the  other  yellow 
and  the  selfed  and  crossed  seeds  on  each  ear  could  be  easily 
distinguished.  The  same  pollen  mixture  was  also  applied  to  a 
third  plant  of  an  inbred  strain  different  from  either  of  the  other 
two  but  more  nearly  related  to  one  than  to  the  other.  The 
average  difference  in  weights  between  the  selfed  and  crossed  seeds 
on  each  ear  are  large.  The  two  out-crossed  lots  of  seeds  on  the 
third  ear  do  not  differ  as  greatly  but  the  heavier  seeds  resulted 
from  the  wider  cross. 

Table  25  gives  a  number  of  averages  of  the  weights  of  seeds 
from  similar  pairs  of  ears  each .  having  selfed  and  reciprocally 
crossed  seeds.  In  every  case  there  is  a  noticeable  increase  in 
weight  as  the  result  of  crossing.  In  Table  26  the  weights  of  the 
out-crossed  seeds  resulting  from  some  of  the  same  pollen  mixtures 
are  given.  Here  again  the  heavier  seeds  are  those  which  have 
resulted  from  the  wider  cross.  A  and  C  are  two  inbred  strains 
derived  from  one  variety  at  the  start  while  B  is  derived  from  a 


62 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 


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HETEROZYGOSIS   AND    SELECTIVE    FERTILIZATION. 


63 


Table  25.    The  immediate  effect  of  pollination  upon  the  weight  of  seeds  of 
maize.     (Selfed  and  reciprocally  crossed  seeds  from  the  same  ears.) 


Pollen 

Pedigree  number  pi 
parent  plant — A 

A 

AXB 

BXA 

B 

Pedigree  number  of 
parent  plant — B 

mixture 

Selfed 

Crossed 

Crossed 

Selfed 

number 

White 

Light   yellow 

Yellow 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

21-3-13-9-7-57-1 

21-3-13-9-7-57-2 

21-3-13-9-7-57-3 

21-3-13-9-7-57-5 

21-3-13-9-7-57-7 

21-3-13-9-7-57-10 

21-3-13-9-7-57-14 

21-3-13-9-7-57-20 

21-3-13-9-7-57-24 

21-3-13-9-7-57-25 

21-3-13-9-7-57-29 

21-3-13-9-7-57-31 

21-3-13-9-7-57-33 

21-3-13-9-7-57-35 

21-3-13-9-7-57-36 

21-3-13-9-7-57-43 

27.0 
20.3 
26.0 
22.2 
26.9 
27.8 
28.0 
30.9 
28.5 
24.6 
32.4 
14.7 
16.5 
19.2 
22.3 
20.6 

32.1 
21.9 
31.1 
24.3 
31.1 
32.4 
30.3 
35.5 
33.0 
29.7 
38.4 
17.3 
18.9 
23.6 
25.1 
22.7 

30.3 
25.2 
30.9 
31.4 
35.2 
29.9 
39.4 
21.6 
29.1 
36.6 
24.1 
24.3 
23.6 
31.3 
36.4 
34.5 

22.3 
21.4 
22.5 
25.3 
28.3 
23.7 
29.5 
21.1 
25.5 
30.1 
19.3 
20.5 
18.5 
25.5 
28.9 
27.3 

14-10-30-4-3-7-11-4 

14-10-30-4-3-7-11-3 

14-10-30-4-3-7-11-10 

14-10-30-4-3-7-11-2 

14-10-30-4-4-2-7-6 

14-10-30-4-3-7-11-1 

14-10-30-4-4-2-7-3 

14-10-30-6-11-3-11-3 

14-10-4-6-4-7-8-5 

14-10-4-6-16-2-12-8 

14-10-30-4-3-7-11-7 

14-10-30-4-3-7-11-8 

14-10-30-4-3-7-11-9 

14-10-30-4-3-7-11-18 

14-10-30-4-4-2-7-14 

14-10-30-4-4-2-7-2 

Average 

Increase  of  crossed 

above  selfed 

Percent  increase 

24.2 

28.0 

3.8 
15.70 

30.2 

5.8 
23.77 

24.4 

Table  26.  The  immediate  effect  of  pollination  upon  the  weight  of  seeds 
of  maize.  (Out-crossed  seeds  resulting  from  some  of  the  same  pollen  mixtures 
used  in  Table  25.) 


Pollen  mixture 
number 

Average  weight  of  seeds  in  centigrams 

Pedigree  number 
of  parent  plant — C 

Cross 
CXA 

Cross 
CXB 

20A-8-5-35-8 

20A-8-5-35-3 

20A-8-5-35-4 

20A-8-5-35-11 
20A-8-5-35-24 
20A-8-5-35-26 

20A-8-5-35-6 

20A-8-5-35-13 
20A-8-5-35-15 
20A-8-5-35-18 
20A-8-5-35-21 
20A-8-5-35-30 
20A-8-5-35-37 

1 

2 
3 
6 
8 
9 
13 

1  ,. 

i 

J 

20.5 
19.7 
25.4 
20.3 
27.3 
25.9 
20.2 

20.1 

23.9 

21.6 

20.2 

21.7 

21.6 

Ave.     21.5         21.5 

24.5 
23.7 
25.0 
22.9 
27.5 
27.7 
20.1 

25.8 

27.5 

20.9 

18.9 

21.2 

21.0 

22.6         22.6 

Increase  of  (CxB)  over  (CxA) 

Percent  increase 

22.7 

24.3 
1.6 
7.05 

64  CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 

different  variety.  All  the  data  taken  together  clearly  show  that 
an  increase  in  endosperm  development  in  maize  is  one  of  the 
common  manifestations  of  heterosis. 

Since  the  crossed  seeds  receive  a  noticeable  impetus  in  develop- 
ment it  seemed  quite  likely  that  the  foreign  pollen  might  be  more 
efficient  in  fertilizing  than  the  self  pollen  and  hence  a  greater 
number  of  crossed  seed  than  selfed  would  be  produced.  Such  is 
not  the  case,  however,  as  an  examination  of  a  large  amount  of 
data  has  shown. 

In  performing  the  mixed  pollinations  no  attempt  was  made  to 
have  more  than  approximately  equal  quantities  of  pollen.  It  is 
impossible  to  get  a  mixture  of  equal  quantities  of  functional 
pollen  because  it  varies  so  in  respect  to  viability.  Since  the  same 
mixture  of  pollen  was  applied  to  both  plants  the  ratio  of  the  seeds 
resulting  from  "yellow"  pollen  to  the  seeds  produced  by  the 
"white"  pollen  should  be  the  same  on  both  ears.  Thus  if  there 
were  no  selective  fertilization  the  percent  of  white  seeds  on  one 
ear  plus  the  percent  of  dark  yellow  seeds  on  the  other,  selfed  seeds 
in  both  cases,  should  .equal  the  sum  of  the  percents  of  the  crossed 
seeds  on  each  ear.  An  excess  of  crossed  seeds  would  then  indicate 
a  selective  fertilization  in  favor  of  the  crossed  pollen.  As  a  small 
excess  of  selfed  seeds  was  obtained  any  selective  fertilization  in 
favor  of  the  foreign  pollen  certainly  did  not  take  place. 

The  numbers  of  the  crossed  and  selfed  seeds,  of  which  the 
weights  are  given  in  Tables  25  and  26,  together  with  a  large 
amount  of  similar  data  are  not  given  here  for  fear  of  unduly 
burdening  this  publication  with  tables  but  they  show,  on  the 
whole,  a  small  excess  of  selfed  seeds  instead  of  crossed  seeds. 
The  results  of  an  experiment  designed  to  test  this  point  in  a  some- 
what different  way  are  given  in  Table  27.  Here  instead  of  taking 
a  mixture  of  pollen  from  two  plants  of  two  different  strains  a 
large  amount  of  pollen  was  collected  from  an  approximately 
equal  number  of  plants  of  two  long  inbred  and  exceedingly  uni- 
form strains  of  maize.  The  two  lots  of  pollen  were  sifted  to 
obtain  pure  pollen  and  equal  quantities  of  each  were  carefully 
measured  out,  thoroughly  mixed  together  and  applied  to  a  number 
of  ears  of  each  of  the  two  strains  which  furnished  the  pollen — 
A  and  B —  and  to  a  third  strain —  C — distinct  from  either. 
Although  the  tassels  were  bagged  on  the  same  day  and  the  pollen 
collected  two  days  later  and  equal  quantities  of  each  taken  there 


HETEROZYGOSIS    AND    SELECTIVE    FERTILIZATION. 


65 


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66  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

was  not  equal  quantities  of  functional  pollen  as  the  number  of 
seeds  given  in  Table  27  show.  The  great  inequality  of  functional 
pollen  may  have  been  due  to  the  fact  that  the  pollen  of  the  B 
strain  was  more  moist  and  tended  to  aggregate  into  a  flocculent 
mass  while  the  pollen  of  the  other  was  perfectly  dry  and  each  grain 
remained  separated  from  the  others.  For  this  reason  it  was  dif- 
ficult to  measure  the  two  lots  of  pollen  equally  and  the  dry  pollen 
clustered  about  the  fine  lumps  of  moist  pollen  when  the  two 
kinds  were  mixed  and  was  probably  first  to  gain  access  to  the 
stigmas.  The  difference  between  the  two  kinds  of  pollen  was  not 
due  to  any  external  differences,  as  far  as  could  be  seen,  and 
indicate  differences  in  the  rate  of  maturing  after  shedding. 

Whatever  may  be  the  cause  of  the  great  difference  in  fertilizing 
power  this  does  not  effect  the  point  under  investigation.  How- 
ever different  the  pollen  may  be,  the  seeds  resulting  from  "  yellow  " 
pollen  should  be  in  the  same  ratio  to  the  seeds  resulting  from  the 
"  white  "  pollen  on  one  ear  as  the  ratio  of  the  same  two  kinds  of 
seeds  on  the  other  ear  within  the  limits  of  the  error  of  random 
sampling  if  there  is  no  selective  fertilization  one  way  or  the  other. 
And  both  these  ratios  should  be  the  same  as  the  third  ratio 
obtained  when  this  same  mixture  of  pollen  is  used  to  produce 
seeds  on  a  plant  of  a  different  variety  of  maize.  Let  us  see  what 
the  figures  given  in  Table  27  show.  Of  the  reciprocal  crosses  and 
selfs  the  proportion,  expressed  as  percent,  is  as  follows: 

Seed  color  carried  by  pollen Yellow  White  Yellow  White 

Type  of  Seeds Selfed  Crossed  Crossed  Selfed 

Actual  proportion  obtained 98.490  :     1.510  ::     96.600  :     3.400 

Closest  perfect  proportion 97.545  :     2.455  ::     97.545  :     2.455 

Deviation 4- .945  -.945  -.945  '  +.945 

The  deviation  from  the  closest  perfect  proportion  is  in  favor 
of  the  selfed  seeds.  This  theoretical  ratio  agrees  very  closely 
with  the  actual  ratio  obtained  from  the  out-crossed  seeds  as 
shown  in  Table  27  although  there  is  considerable  difference 
in  the  results  from  the  different  ears.  Letting  S  stand  for  selfed 
and  C  for  crossed  the  probable  error  of  the  determination 
S  .  .6745  /(SHC)  •-    ";       .  S 

uc     1S  ±     sT~c    v  STC  '      The  fractlon    sTc 

gives  the  percent  of  selfed  seeds  and  the  probable  error  is  stated 

C 

as  percent.    Likewise  the  fraction  = -^  gives  the  percent  of 

b  -7-   V_' 


HETEROZYGOSIS   AND  ^SELECTIVE    FERTILIZATION.  67 

crossed  seeds  and  the  probable  error  is  the  same  as  for  the  percent 
of  selfed  seeds. 

This  same  experiment  was  repeated  with  about  the  same 
number  of  plants  with  the  result  of  a  similar  excess  of  selfed 
seeds  greater  than  would  be  expected  from  the  probable  error 
on  the  assumption  that  there  is  no  selective  fertilization.  Does 
this  mean  that  there  is  a  selective  fertilization  in  favor  of  a  plant's 
own  pollen  and  that  the  plant  discriminates  against  foreign  pollen 
even  though  the  seeds  resulting  from  that  foreign  pollen  are 
greatly  increased  in  size,  weight,  viability  and  the  rate  of  growth 
of  the  ensuing  plants?  Unless  there  has  been  a  constant  error  in 
classifying  the  seeds  this  seems  to  be  the  necessary  conclusion  to 
be  drawn  from  the  results  so  far  given  by  maize.  A  sufficient 
number  of  plants  will  be  grown  from  this  seed  to  determine 
definately  whether  or  not  there  has  been  any  error  in  the  separa- 
tion of  the  seeds  so  that  this  question  can  be  answered  with  a 
high  degree  of  certainty. 

In  the  meantime  there  is  little  doubt  but  that  there  is  no  great 
selective  fertilization  in  favor  of  cross-pollination,  if  any,  however 
much  that  cross-pollination  may  benefit  the  resulting  seeds  and 
the  plants  grown  from  them.  If  this  is  true  crossing  is  without 
effect  until  the  zygote  is  formed  at  the  time  of  the  union  of  the 
male  and  female  nuclei. 

In  a  consideration  of  selective  fertilization  it  should  be  remem- 
bered that  there  are  two  different  conditions  which  may  be  included 
in  the  term  selective  fertilization.  One  may  be  said  to  be  the 
selection  of  different  germ-plasms;  the  other  the  selection  of 
different  cytoplasms.  For  example  a  heterozygous  plant  produces 
pollen  grains  with  different  germinal  compositions  but  all  enclosed 
in  the  same  cytoplasm.  On  the  other  hand  pollen  from  different 
plants  may  differ  in  the  nature  of  the  cytoplasm  as  well  as  in 
hereditary  factors  carried  in  the  nuclear  material.  East  and 
Park  ('18)  have  demonstrated  that  in  tobacco  there  is  no  selective 
fertilization  between  gametes  coming  from  one  plant  although 
the  pollen  grains  differ  in  factors  which  determine  fertility  or 
sterility  of  the  ensuing  plants.  The  case  is  quite  similar  to  that 
of  the  shape  of  pollen  grains  in  peas  which  may  be  either  all 
round  or  all  cylindrical  according  to  the  germinal  composition 
of  the  sporophyte  which  produced  them  and  not  according  to 
the  factors  which  they  carry.     Where  pollen  grains  differ  both 


68  CONNECTICUT?   EXPERIMENT   STATION   BULLETIN    207. 

in  the  factors  which  they  carry  and  in  the  plants  from  which 
they  come,  as  is  the  case  with  these  experiments  with  maize, 
the  conditions  are  quite  different.  It  would  not  be  surprising 
that  there  should  be  selective  fertilization  in  one  case  and  not 
in  the  other.  East  and  Park  have  shown  that  a  tobacco  plant 
which  was  self-sterile,  pollinated  with  a  mixture  of  its'  own  and 
pollen  from  another  plant  with  which  it  was  fertile,  gave  all 
crossed  seeds — a  maximum  of  selective  fertilization. 

Darwin  ("  Cross  and  Self  Fertilization  ")  found  that  there  was  a 
selective  fertilization  in  favor  of  foreign  pollen  in  different  plants. 
Many  of  Darwin's  experiments,  however,  were  made  in  such  a 
way  as  to  be  open  to  doubt  whether  or  not  he  really  did  obtain 
such  an  effect.  His  experiments,  in  applying  foreign  pollen 
sometime  after  self-pollination  had  taken  place,  in  which  he 
obtained  in  some  cases  many  or  all  apparently  crossed  progeny, 
are  open  to  other  interpretations.  The  purity  of  the  plants 
pollinated  was  not  known.  External  conditions  influencing 
fertilization  were  not  guarded  against.  Taken  as  they  stand, 
however,  his  experiments  with  Mimulus,  Iberis,  Brassica,  Raphi- 
nus,  Allium  and  Primula  do  indicate  that  in  these  plants  there 
may  be  a  selective  fertilization  in  favor  of  foreign  pollen.  It  is 
to  be  expected  that  plants  which  show  partial  self-incompati- 
bility would  show  selective  fertilization  when  a  mixture  of  self 
and  foreign  pollen  was  applied.  In  maize,  however,  as  mentioned 
before,  the  sterility  shown  is  in  the  nature  of  pollen  and  ovule 
abortion,  and  whenever  well  formed  pollen  is  produced  it  seems 
to  be  able  to  fertilize  equally  any  plants  if  not  too  distinct  in  type. 
A  distinction  should  be  made,  then,  between  self-fertile  plants  and 
self -sterile  plants  when  dealing  with  selective  fertilization. 

Hyde  ('14)  has  shown  clearly  that  in  Drosophila  both  males 
and  females  of  inbred  lines  are  more  productive  of  offspring 
when  mated  to  an  individual  of  a  different  line  than  when  mated 
to  one  of  their  own.  Both  males  and  females,  therefore,  produce 
more  functional  gametes  than  are  utilized  when  individuals  of 
the  same  inbred  lines  are  paired.  Hence  a  female,  impregnated 
with  a  mixture  of  two  kinds  of  spermatozoa  from  the  same  and 
from  different  lines  would  produce  more  hybrid  progeny  than 
inbred  progeny  even  if  equal  quantities  of  both  types  of  sperma- 
tozoa were  available  for  fertilization.  In  other  words  there  would 
be  selective  fertilization  in  favor  of  cross-fertilization. 


LONGEVITY,  HARDINESS   AND    VIABILITY.  69 

Whether  or  not  there  may  be  a  similar  condition  in  other  animals 
I  do  not  know.  Even  in  Drosophila,  fertilization  by  the  two  types 
of  sperm  may  take  place  equally,  and  a  greater  proportion  of 
close-fertilized  eggs,  than  cross-fertilized,  fail  to  hatch,  due  to 
lesser  vigor  or  lethal  factors.  In  Hyde's  experiments  the  type 
of  fertilization  had  no  marked  effect  on  the  number  of  eggs  laid, 
only  on  the  percentage  which  hatched. 

In  maize,  and  possibly  all  plants  which  show  no  self-incompati- 
bility, the  fact  seems  clear  that  crossing  is  wholly  without  effect 
until  the  fertilization  process  is  completed. 

Although  there  is  apparently  no  effect  of  crossing  in  maize 
until  the  zygote  is  formed,  such  an  effect  is  apparent  immediately 
afterwards.  In  addition  to  the  increase  in  endosperm  development 
there  is  also  an  increase  in  the  vigor  of  the  embryo.  Whether  or 
not  the  size  of  the  embryo  in  the  seed  is  increased  has  not  been 
actually  determined,  other  than  by  inspection,  but  it  undoubtedly 
is,  along  with  the  endosperm.  When  crossed  and  selfed  seeds 
from  the  same  ear,  grown  on  a  plant  which  has  been  inbred 
previously  for  several  generations,  are  planted  a  striking  difference 
is  soon  apparent.  The  crossed  seedlings  appear  from  one  to  two 
days  before  the  selfed  seedlings  and  may  be  two  or  three  inches 
above  ground  before  any  of  the  selfed  plants  begin  to  appear. 
(See  Plate  Xlb).  From  then  on  the  superiority  of  the  crossed 
over  the  selfed  plants  increases  rapidly  as  shown  by  the  curves 
in  Figure  III. 


The  Effect  of  Heterozygosis   upon   Longevity,  Hardiness 

and  Viability.  - 

An  increased  longevity,  viability  and  endurance  against  un- 
favorable climatic  conditions  have  been  frequently  noted  in 
hybrids.  Kolreuter  and  Wiegmann  both  mention  this  fact. 
Gartner  in  his  book  "Bastarderzeugung  im  Pflanzenreich"  devotes 
considerable  attention  to  this  feature.  Under  the  heading 
"  Ausdauer  und  Lebenstenacitat  der  Bastardpflanzen"  he  makoo 
the    following    statements. 

"  There  is  certainly  no  essential  difference  between  annual  and  biennial 
plants  and  between  these  and  perennials  in  regard  to  their  longevity; 
for  it  is  not  seldom  that  different  individuals  of  the  same  species  have  a 
longer   life    at  times  as,  for  example,  Draba  verna,  which  has  annual  and 


70  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

biennial  forms;  the  longevity  of  a  plant  furnishes  thereby  no  specific 
differences  but  signifies  at  most  a  variability  as  Prof.  W.  D.  I.  Koch 
has  shown.  However,  in  hybrids  this  difference  deserves  special  con- 
sideration. In  most  hybrids  an  increased  longevity  and  greater  endur- 
ance can  be  observed  as  compared  with  their  parental  races  even  if  they 
come  into  bloom  a  year  earlier.  The  union  of  a  annual,  herbaceous 
female  plant  with  a  perennial,  shrubby  species  through  hybridization 
does  not  shorten  the  life  cycle  of  the  forthcoming  hybrid  as  the  union 
of  Hyoscyamus  agrestis  with  niger,  Nicotiani  rus'tica  with  perennis, 
Calceolaria  plantaginea  with  rugosa  shows,  and  so  also  in  reciprocal 
crosses  when  the  perennial  species  furnishes  the  seed  and  the  annual 
species  supplies  the  pollen,  as  Nicotiana  glauca  with  Langsdorfii,  Dianthus 
caryophyllus  with  chinensis,,  Malva  sylvestris  with  Mauritiana  or  biennials 
with  perennials  and  reciprocally  as  Digitalis  purpurea  with  Ochroleuca  or 
lutea  and  lutea  with  purpurea  or  ochroleuca  with  purpurea.  From  the 
union  of  two  races  of  different  longevity  comes  usually  a  hybrid  into 
which  the  longer  life  of  one  or  the  other  of  its  parent  races  is  carried 
whether  it  comes  from  the  male  or  female  parent  species." 

Many  more  instances  are  given  by  Gartner  from  his  own  ob- 
servations and  those  of  others  to  enable  him  to  reach  the  following 
conclusion : 

"  These  examples  support  the  statement  of  Kolreuter's  that  the  longer 
life  of  hybrid  plants  is  to  be  counted  among  their  usual  properties." 

With  regard  to  the  resistance  of  hybrids  to  unfavorable  weather 
conditions  he  goes  on  to  say: 

"  With  their  longevity  stands,  in  the  closest  relation,  the  fairly 
common  property  of  hybrids  to  withstand  lower  temperatures  than  their 
parental  races  without  injury  to  their  growth  and  vegetative  life.  Kol- 
reuter  first  observed  that  Lyciutn  barbara-afrum  in  south  Germany 
withstood  the  winter  in  the  open  field;  although  Lycium  afrum  must  be 
wintered  over,  at  least,  in  a  cold  frame.  The  cross  of  Nicotiana  Tabaco- 
undulata,  according  to  Sageret  in  France  had  an  increased  life,  although 
in  a  protected  place,  in  open  field.  W.  Herbert  reports  that  Rhododen- 
dron altaclarae,  which  is  a  hybrid  union  of  R.  pontica-cantawbiense  9  with 
the  very  sensitive  Nepalense  arboreum  coccineum  c?,  has  been  grown  in 
the  open  in  England;  also  Robert  Sweet  confirms  the  same  result  by 
a  hybrid  crinum  and  many  other  hybrids  of  bulbous  plants  grown  in 
open  field  whose  parental  species  must  be  grown  in  the  hothouse. 

"  Lobelia  syphilitica-cardinalis  wintered  over  with  a  light  covering 
in  the  winter  of  1832-1833  with  5°F  in  open  field.  Lychnicucubalus 
albus  and  ruber  lasted  three  years  in  open  field  although  cucubalus 
viscosus  in  south  Germany  did  not  survive  in  open  field.  All  hybrids 
of  genus  coccineum  stood  over  the  winter  of  1842-1843  with  5°F.  in  the 
open,  although  the  pure  species  seldom  lives  through  our  usual  winters 
of  43°  to  9.5°  F.     Prof.  Wiegmann  reports  similar  results. 


LONGEVITY,  HARDINESS   AND    VIABILITY.  71 

"  Very  frost  sensitive  species  of  Nicotiana  and  their  hybrids  did  not 
withstand,  under  the  same  conditions,  such  low  temperatures  as  the 
afore-mentioned  plants;  but  we  have  flowered  and  carried  over  part 
of  them  wherever  they  were  well  covered  with  snow,  for  example,  N. 
quadri-valvis  glutinosa,  rustica-quadrivalvis,  these  withstood  25°  F.  and 
yet  have  continued  blooming  although  N.  glutinosa,'  quadrivalvis,  panicu- 
late/,, Tabacum  and  rustica  were  already  frozen  by  32°  F.  Moreover  other 
crosses  of  very  sensitive  and  tender  species  of  this  genus  as  paniculata- 
Langsdorfii,  vincaeflora-Langsdorfii,  vincae-flora-quadrivalvis  have  been 
carried  over  in  an  active  growing  condition  two  to  three  years,  and 
glauca-Langsdorfii  three  years  in  a  cold  house  with  39°  to  42°.  The 
hybrid  N.  paniculatarustica-paniculata  was  kept  over  in  a  cold  house  in 
the  cold  winter  of  1839-40  but  its  leaves  were  yellow.  Among  all  the 
species  of  this  genus  the  cross  of  N.  suaveolenti-macrophylla  showed 
itself  to  be  the  most  hardy.  On  the  16th  of  October  of  its  first  year 
(1828)  its  top  was  frozen  but  it  did  not  suffer  from  this,  and  12  days  later 
put  out  a  new  shoot  from  the  root  and  its  leaves  lasted  through  the  winter 
in  a  cold  house  in  a  fresh,  green  condition  although  the  other  species 
were  yellow  and  this  plant  was  the  first  to  start  into  growth  in  the  spring. 
The  same  endurance  Sageret  observed  in  Nicotiana  suaveolenti-virginica. 
All  these  plants  in  the  last  year. of  their  vegetative  life  seemed  to  die 
off  more  as  the  result  of  the  unfavorableness  of  the  weather  than  of  old 


Exceptions  are  noted  by  Gartner  in  that  some  species  which 
were  not  resistant  to  cold  did  not  give  resistant  hybrids.  In 
many  cases  the  hybrids  were  weak  because  of  the  distant  re- 
lationship of  the  parental  races. 

Sargent  ('94)  reports  a  remarkably  vigorous  and  hardy  hybrid 
tree  supposed  to  be  a  cross  of  the  tender  English  walnut,  Juglans 
regia  and  the  common  butternut  Juglans  cinerea.  He  says: 
p.  434 

"My  attention  was  first  called  to  the  fact  by  observing  that  a  tree  which 
I  had  supposed  was  the  so-called  English  walnut — Juglans  regia,  in  the 
grounds  connected  with  the  Episcopal  School  of  Harvard  College  at 
Cambridge,  was  not  injured  by  the  cold  of  the  severest  winters,  although 
Juglans  regia  generally  suffers  from  cold  here —  and  rarely  grows  to  a 
large  size.  This  individual  is  really  a  noble  tree;  the  trunk  forks  ab  u 
five  feet  above  the  surface  of  the  ground  into  limbs  and  girths,  at  the 
point  where  its  diameter  is  smallest,  fifteen  feet  and  two  inches.  The 
divisions  of  the  trunk  spread  slightly  and  form  a  wide,  round-topped 
head  of  pendulous  branches  and.  unusual  symmetry  and  beauty,  and 
probably  sixty  to  seventy  feet  high." 

Heterosis  is  also  shown  in  a  resistance  to  bacterial  and  fungus 
diseases.    Some  of  the  inbred  strains  of  maize  are  very  susceptible 


72 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 


to  the  bacterial  leaf-wilt  and  in  some  years  at  the  end  of  the 
season  all  the  plants  of  these  strains  appear  as  if  they  had  been 
scorched  by  fire  while  other  strains  in  adjoining  rows  are  un- 
touched. Other  strains  have  quite  a  large  percentage  of  plants 
attacked  by  smut.*  Crosses,  however,  of  these  susceptible  strains 
with  those  which  are  not  affected  by  these  parasitic  organisms 
are  only  slightly  or  not  at  all  affected. 


Table  28.     Susceptibility   to  smut    (  Ustilago  zeae)   of  a  non-inbred 

VARIETY  OF  MAIZE,  SEVERAL  INBRED  STRAINS  DERIVED  FROM  THIS 
VARIETY  AND  THE  FIRST  AND  SECOND  GENERATION  CROSSES 
BETWEEN  THE  MOST  SUSCEPTIBLE  AND  THE  LEAST  SUSCEPTIBLE 
STRAINS. 


Percent  of  plants  affected 

Total 

number 

of  plants 

grown 

Total 
percent 
of  plants 
affected 

Plot  I 

Plot  II 

Plot  III 

1 

l_9_l-2-4-6-7-5  

o' ' 

2.17 

8.79 
0 

'  .27 
.35 
10.16 
0 

2.48 

1.75 

.56 

0 
5.77 

0 

0 
5.15 

114 

596 
408 
950 
992 
439 
97 

1.75 
.34 

1-7-1-2-2-9-2-1  

.49 

1-7-1-1-1-4-7-5  

9.79 

l_6_l_3_4_4-4_2  

0 

(1-6-1-3)  X (1-7-1-1^..  . 
(1-6-1-3)  X(l-7-l-l)F„..  . 

2.28 
5.15 

In  Table  28  are  given  the  per  cent,  of  plants  affected  by  smut 
(  Ustilago  zea,  Beck.  Ung.)  of  the  original,  non-inbred  Learning 
variety  of  maize  previously  spoken  of  and  four  inbred  strains 
derived  from  this  variety  by  ten  or  eleven  generations  of  self- 
pollination.  Seed  of  the  four  inbred  strains  was  planted  in  three 
rather  widely  separated  plots  in  the  same  field  in  1917.  Two  of 
the  strains  showed  only  a  small  infection  by  this  parasite;  one 
showed  about  10  per  cent  infection  and  one  had  not  a  single  plant 
affected  in  all  three  plots  in  a  total  of  nearly  one  thousand  plants. 
Since  the  differences  which  these  four  strains  show  are  fairly  con- 
sistent in  the  different  places  grown  it  can  hardly  be  doubted  but 
that  segregation  of  susceptibility  to  parasitism  has  occurred  in 
the  inbreeding  process.  The  first  generation  hybrid  between  the 
most  resistant  and  the  most  susceptible  strain  was  free  from  smut 
in  one  plot  and  but  slightly  affected  in  another.  The  second 
generation  hybrid  grown  side  by  side  with  first  generation  showed 


LONGEVITY,  HARDINESS   AND    VIABILITY. 


73 


considerably  more  infection  although  the  number  of  plants  grown 
was  small.  This  is  fairly  good  evidence  that  resistance  to  smut 
in  maize  tends  to  dominate  in  crosses  between  plants  which  differ 
in  this  respect.    - 

Tisdale,  according  to  L.  R.  Jones  ('18)  also  finds  that  in  flax 
disease  resistance  tends  to  be  dominant  although  the  hybrids 
are  more  or  less  intermediate  in  this  respect  and  the  method  of 
inheritance  is  rather  complex.  Biff  en  ('12),  on  the  other  hand, 
concluded  that  the  resistance  to  rust  in  wheat  was  recessive. 
Likewise,  Weston  ('18)  states  that  maize  and  teosinte-maize  hy- 
brids are  extremely  susceptible  to  a  downy  mildew  (Peronospora 
Maydis,  Rac.)  in  Java  and  other  places,  although  teosinte  (Euch- 
laena  mexicana,   Schrad.)  is  immune. 

Data  from  another  source  have  been  obtained  from  the  garden 
radish  (Raphanus  sativus,  L.).  A  white-rooted  variety  of  radish 
was  allowed  to  go  to  seed  alongside  a  red-rooted  radish.  Seed 
collected  from  the  white-rooted  plants  was  sown  thickly  in  a  flat 
and  when  they  came  up  it  was  seen  that  a  number  of  the  seedlings 
were  crossed  from  their  red  -colored  stems.  The  seedlings  were 
quite  badly  attacked  by  the  "damping-off"  fungus  and  large 
numbers  of  them  were  killed,  but  a  far  less  number  of  the  crossed 
seedlings  were  affected  as  shown  by  the  decay  of  the  tissues  at. 
the  base  of  the  stem.    The  figures  obtained  are  given  in  Table  29. 


Table  29.     Comparative  susceptibility  to  "  damping-opf 
of  selfed  and   crossed  radish  seedlings. 


White  Seedlings,  Selfed 

Red  Seedlings,  Crossed 

Variety  of  Radish 

Number 
grown 

Number 
affected 

Percent 
affected 

Number 
grown 

Number 
affected 

Percent 
affected 

Short,  white. .  . 
Long,  white. .  . 

349 
76 

142 

28 

40.7 
36.8 

30 

7 

4 
0 

13.3 
0 

Gernert  ('17)  reports  a  case  of  immunity  to  aphis  attack  .of 
teosinte-maize  hybrids  in  which  the  maize  parent  was  badly 
infested  whereas  the  teosinte  parent  and  the  hybrid  entirely 
escaped  injury. 

Together  with  these  manifestations  of  heterosis  in  its  influence 
on  hardiness  there  is  an  increase  in  the  viability  of  crossed  seeds 
as  compared  to  selfed  seeds  from  the  same  ears  as  shown  in  Table 


74 


CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 


Table  30.     The  effect  of  heterozygosis   upon   germination — a  compar- 
ison OF  crossed  and  selfed  seeds  from  the  same  ears  of  maize 


Ml  > 
■  3    o 

o 

s 

"pi 

o 

T3 

TO 

X) 
CD 

TJ 
O 

TO 

CD 
(O 

TO 

-c 

CD 
O 

-a 

CD 

\t   CD 
TO    > 
TOO 

Eh 

Ot3 
CD 

Pedigree  number 

Pedigree  number 

in  m 
eg 

13 
CD 

CD 

c 
u 

- 

TO 

CD    fi 

£'3 

of  female  parent 

of  male  parent 

a  m 

■M    O 

C  "  « 

2"S  m 

<d  a 

•ss 

®12 

ce 

c  S 

CD    M 

M  to 

OT3 

*■«  ^ 

.      »  03 

C 

c  S 

CD   S* 

O    CD 

o  ~ 
**  CD 
'  O       ^ 

T.  ~     t 

TO   ^<~ 

gS'S 

^    Oi    IB 

P4 

fc 

fc 

£ 

ft      ■ 

ft 

H 

•    21-3-13-9-7-57-13 

14-10-30-6-2-13-5-13 

8.2 

121 

24 

46 

19.8 

38.0 

18.2 

21-3-13-9-7-57-17 

14-10-30-6-11-3-11-17 

18.8 

39 

26 

37 

66.7 

94.9 

28.2 

21-3-13-9-7-57-21 

14-10-30-6-11-3-11-4 

.7.1 

32 

28 

29 

87.5 

90.6 

3.1 

21-3-13-9-7-57-38 

14-10-30-4-4-2-7-38 

68.0 

22 

14 

22 

63.6 

100.0 

36.4 

21-3-13-9-7-57-39 

14-10-30-4-4-2-7-7 

18.3 

33' 

16 

26 

48*5 

78.8 

30.3 

21-3-13-9-7-57-54 

14-10-30-4-4-2-7-7 

3.9 

97 

19 

27 

19.6 

27.8 

8.2 

21-3-13-9-7-57-58 

14-10-4-6-4-7-8-15 

8.8 

100 

43 

68 

43.0 

68.0 

25.0 

21-3-13-9-7-57-59 

14-10-4-6-4-7-8-10 

10.0 

12 

9 

12 

75.0 

100.0 

25.0 

21-3-13-9-7-57-63 

14-1Q-4-6-4-7-8-29 

17.5 

31 

26 

30 

83.9 

96.8 

12.9 

21-3-13-9-7-57-64 

14-10-4-6-4-7-8-29 

13.3 

14 

9 

14 

64.3 

100.0 

35.7 

21-3-13-9-7-57-65 

* 

13.5 

47 

41 

45 

87.2 

95.7 

8.5 

14-10-30-4-4-2-7-12 

21-3-13-9-7-57-38 

8.3 

87 

84 

86 

96.6 

98.9 

2.3 

Total 

16.3 

635 

339 

442 

53.4 

69.6 

16.2 

*  Seeds  crossed  but  number  of  parent  unknown. 


30.  Seeds  which  were  secured  from  some  of  the  mixed  pollinations, 
reported  previously,  were  sown  in  flats.  Without  exception  the 
crossed  seeds  showed  a  higher  percentage  of  germination  than 
the  selfed  seeds  from  the  same  ears  as  can  be  seen  in  Plate  Xlb. 
These  seeds  were  planted  two  months  after  ripening.  Whether 
or  not  an  increase  in  age  would  show  greater  differences  in  viability 
is  not  known  but  it  is  quite  likely  that  the  difference  might  be- 
come even  greater  with  age  up  to  a  certain  point.  The  low  germi- 
nation of  both  crossed  and  selfed  seeds  in  some  of  the  ears  was 
due  to  the  fact  that  they  were  moldy  on  account  of  late  ripening 
and  damp  weather. 

The  increased  vegetative  vigor  as  manifested  by  an  increased 
facility  of  vegetative  propagation  in  hybrids  has  been  repeatedly 
spoken  of.  Kolreuter,  Wiegmann,  Sageret  and  Focke  make  a 
special  mention  of  this  phenomenon. 

Moreover  there  is  no  positive  evidence  that  plants  which  are 
propagated  vegetatively  lose  *  any  of  their  hybrid  vigor  which 


LONGEVITY,  HARDINESS   AND    VIABILITY.  75 

they  may  have,  no  matter  how  many  generations  of  asexual  re- 
productions take  place.  Undoubtedly  most  varieties  of  culti- 
vated fruits,  flowers,  ornamental  plants  and  field  crops  which  are 
commonly  propagated  vegetatively,  owe  their  excellence  in  part 
to  heterosis. 

From  time  to  time  the  supposed  degeneration  of  plants  in  long- 
continued  vegetative  propagation  has  been  much  disputed. 
Knight  ('99)  and  Van  Mons  ('36)  contended  that  they  did  degen- 
erate, but  Lindley  ('52)  reviewing  Knight's  work  thought  that 
the  evidence  did  not  support  such  a  view.  Gartner  states  that 
the  characteristics  of  a  hybrid  do  not  change  throughout  the 
whole  life  cycle  of  the  individual,  even  when  it  is  propagated  and 
disseminated  by  buds,  cuttings  or  layers. 

Darwin  believed  that  a  degeneration  took  place  largely  for  the 
same  reason  that  he  thought  long  continued  self-fertilization  was 
injurious.  Asa  Gray  ('76),  in  reviewing  Darwin's  opinions  on 
this  matter,  says  (p.  347) : 

"The  conclusion  of  the  matter,  from  the  scientific  point  of  view  is,  that 
sexually  propagated  varieties  of  races,  although  liable  to  disappear  through 
change,  need  not  be  expected  to  wear  out  and  there  is  no  proof  that  they 
do,  but  that  non-sexually  propagated  varieties,  though  not  especially 
liable  to  change,  may  theoretically  be  expected  to  wear  out,  but  to  be  a 
very  long  time  about  it." 

Gray,  however,  cites  cases  of  horticultural  varieties  propagated 
since  the  time  of  the  Romans  with  no  apparent  loss  of  vigor. 
Whitney  ('12a,  b,  c)  and  A.  F.  Shull  ('12b)  believe  that  an  actual 
degeneration  takes  place  in  parthenogenetic  reproduction  in  the 
rotifiers.  The  work  of  Enriques  ('07),  Woodruff  ('11)  and  Jennings 
('12)  on  Paramecium  proves  almost  beyond  doubt  that  there  is 
no  degeneration  in  this  organism  although  reproduction  by 
fision  in  the  infusoria  may  be  considerably  different  from  vegetative 
propagation  in  the  higher  plants.  Hedrick  ('13),  from  the  evi- 
dences of  long-continued  varieties  of  fruits,  and  East  ('08)  working 
with  potatoes  and  reviewing  extensively  the  whole  question  be- 
lieve that  there  is  no  evidence  that  a  real  degeneration  takes  place 
which  cannot  be  accounted  for  on  the  basis  of  the  accumulation 
of  disease  or  other  external  effects.  East  ('10),  however,  suggested 
that  such  a  degeneration,  if  ever  proven,  might  be  accounted  for 
on  the  basis  of  a  decreasing  effect  of  the  physiological  stimulation 
assumed  to  be  derived  from  heterozygosity.     A.  F.  Shull  (12a) 


76  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

holds  a  similar  opinion.  From  the  nature  of  the  problem  it  can 
hardly  be  settled  satisfactorily  one  way  or  the  other  by  experi- 
mental means.  As  it  stands  at  present  there  is  no  clear  evidence 
that  there  is  a  degeneration  in  long  continued  asexually  propagated 
plants.     The  burden  of  proof  rests  with  the  positive  side. 

The  Effect  of  Heterozygosis  upon  the  Time  of  Flowering 
and  Maturing. 

Many  investigations  have  indicated  that  there  is  a  hastening 
of  the  time  of  maturity  due  to  heterozygosis.  That  there  is  an 
acceleration  in  the  rate  of  growth  is,  of  course,  evident  from  the 
great  increase  in  size  shown  by  hybrids  grown  in  the  same  season 
with  their  parents.  There  is,  moreover,  considerable  evidence 
from  previous  work  and  from  the  data  to  be  given  here  to  show 
that  hybrids  not  only  grow  to  a  larger  size  but  complete  their 
growth  in  a  shorter  time  than  the  parents  take  to  complete  a 
smaller  amount  of  growth.  In  other  words,  heterozygosis  tends 
to  hasten  the  time  of  maturity  as  well  as  to  increase  size. 

The  investigations  of  Kolreuter,  Gartner,  Focke  and  Darwin 
show  a  large  number  of  specie-  and  variety-crosses  wherein  the 
hybrid  flowers  before  either  pi  the  parents.  Both  Kolreuter  and 
Gartner  give  instances  of  perennials  which  commonly  bloom  in 
the  second  or  third  year  whose  hybrids  bloom  in  the  first  year. 

The  most  extensive  observations  bearing  on  this  relation  of 
heterosis  to  time  of  flowering  are  those  given  by  Darwin  in  his 
"Cross  and  Self  Fertilization  in  the  Vegetable  Kingdom."  He 
gives  the  time  of  flowering  of  28  crosses  between  different  strains 
within  many  different  species- — which  show  positive  evidence  of 
hybrid  vigor.  Of  these  28  crosses  81  per  cent,  flower  before  the 
parents.  Four  cases  are  given  where  the  crosses  are  less  vigorous 
than  the  parents  and  in  each  of  these  the  parents  flowered  first- 
Recent  experiments  in  hybridization  show,  almost  without 
exception,  that  crosses  which  result  in  an  increase  in  vigor  also 
result  in  a  hastening  of  the  time  of  flowering.  One  exception  to 
this  statement  must  be  noted  in  the  cross  between  a  large  dent 
and  a  small  pop  variety  of  corn  repoited  by  Emerson  and  East 
('13).  This  cross  showed  distinct  evidence  of  hybrid  vigor  in  an 
increase  in  internode  length  over  that  of  both  parents.  The 
parents  differed  in  time  of  flowering  by  25  days.  The  first  genera- 
tion of  the  cross  grown  the  same  year  as  the  parents  was  "distinctly 


TIME    OF   FLOWERING   AND   MATURING.  77 

intermediate"  in  time  of  flowering.  There  was  an  increase  in  the 
rate^of  growth  necessarily  as  the  plants  were  larger  than  the  av- 
erage of  the  parents. 

Data  bearing  upon  the  relation  of  heterozygosis  to  the  time  of 
maturing  has  been  secured  from  two  different  plants,  tomatoes 
and  corn.  A  large  part  of  the  data  on  tomatoes  was  collected  by 
Prof.  H.  K.  Hayes,  now  at  the  Minnesota  College  and  Station. 

Four  commercial  varieties  of  tomatoes  were  successively  self- 
pollinated  for  four  years.  Two  first  generation  crosses  between 
these  varieties  were  grown  in  each  of  the  four  years  and  compared 
as  to  yield  of  fruit  and  time  of  production  with  the  two  selfed 
parents.  In  every  case  the  same  plants  which  were  used  to  pro- 
duce the  selfed  seed  for  the  next  generation  were  also  used  to 
make  the  crosses.  For  this  reason  and  because  tomatoes  are 
naturally  self-pollinated  and  are  hence  in  a  homozygous  condition 
the  first  generation  crosses  can  be  compared  strictly  with  their 
parents. 

From  thirty  to  fifty  plants  of  each  variety  and  cross  were  grown 
each  year.  The  fruit  was  picked  as  it  ripened  at  intervals  of  from 
3  to  5  days  and  the  average  production  per  plant  was  determined. 
One  of  the  crosses  was  between  varieties  which  had  approximately 
the  same  time  of  ripening.  This  first  generation  cross  did  not  ex- 
ceed, in  total  yield,  the  average  of  the  two  parents  and  did  not 
differ  from  them  in  respect  to  time  of  production. 

The  other  cross,  however,  yielded,  each  year,  an  average  of  16 
percent  above  the  better  parent.  The  two  varieties  used  in 
making  this  cross  differed  in  time  of  production  by  an  average 
of  five  days.  The  first  generation  cross  while  yielding  16  percent 
more  than  the  late  parent  was  each  year  fully  as  early  as  the 
early  parent.  Although  the  difference  in  time  of  production 
between  these  varieties  is  small  the  consistent  results  obtained  in 
four  successive  years  are  certainly  significant. 

Similar  results  were  secured  with  sweet  corn.  A  first  generation 
cross  between  an  early  variety  of  sweet  corn,  Golden  Bantam  and 
a  late  variety,  Evergreen,  was  grown  in  1916  together  with  the 
two  parental  varieties  and  compared  in  time  of  flowering,  number 
of  ears  per  plant  and  in  height.  They  were  all  planted  at  the 
same  time  but  rather  late  in  the  season  so  that  the  early  and  late 
varieties  bloomed  at  more  nearly  the  same  time  than  is  usually 
the  case.     About  half  of  the  plants  of  the  early  variety  were 


78  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

showing  silks  before  the  late  variety  commenced  to  silk  ou  l 
The  first  generation  cross  was  slightly  earlier  than  the  early 
parent  in  producing  silks.  The  cross  was  noticeably  affected  by 
vigor  of  crossing  in  that  it  was  fully  as  tall  as  the  taller  parent 
and  averaged  more  ears  per  plant  than  either  parent  although 
the  ears  were  not  as  large  as  those  of  the  Evergreen  variety. 

Much  more  extensive  and  authoritative  data  have  been  secured 
from  a  comparison  of  inbred  strains  of  corn  with  their  first  genera- 
tion crosses.  Forty-two  strains  of  corn  which  had  been  continu- 
ously selfed  for  from  5  to  11  generations  and  100  first  generation 
crosses  representing  different  combinations  between  these  selfed 
strains  were  grown  under  the  same  conditions  as  to  time  of 
planting  and  culture.  Both  the  inbred  strains  and  their  crosses 
were  exceedingly  uniform  in  time  of  flowering  and  maturing. 
All  the  plants  in  any  selection  flowered  and  matured  within  a  few 
days.  About  60  plants  of  each  were  grown.  At  intervals  of  one 
week  during  the  flowering  season  the  number  of  selections  of  the 
selfs  and  crosses  which  had  flowered  by  that  time  were  noted. 
Similarly  at  the  end  of  the  season  the  selections  which  were  mature 
were  noted  at  intervals.  Although  the  time  of  maturity  can  not 
be  so  definitely  determined  as  the  time  of  flowering  all  the  plants 
in  a  selection  were  uniform  in  this  respect.  For  the  flint  varieties 
the  glazing  of  the  ears  and  for  the  dent  varieties  the  denting  of 
the  kernels  were  taken  as  indications  of  maturity.  The  crosses 
yielded,  on  the  average,  180  per  cent  more  than  their  parents. 

Together  with  this  increase  in  the  amount  of  growth  there 
was  a  noticeable  hastening  of  both  the  time  of  flowering  and 
maturing.  In  time  of  flowering  the  crosses  were  four  days  and 
in  maturing  eight  days  earlier  than  the  average  of  their  parents. 
Since  the  crosses  gave  a  large  increase  in  the  total  amount  of 
growth  and  produced  this  growth  in  a  somewhat  shorter  time  than 
their  inbred  parents  it  is  all  the  more  evident  that  heterozygosis 
increases  the  rapidity  of  growth.     See  Plates  VII  a  and  b. 

The   Relation   of  the   Effects   of  Heterozygosis  and   of 
the  Environment. 

East  ('16)  has  stated  that  heterozygosis  "  affects  a  result 
comparable  to  favorable  external  conditions."  In  a  cross  between 
two  varieties  of   Nicotiana  he  found  that  the  first  generation 


EFFECTS    OF   HETEROZYGOSIS   AND   THE    ENVIRONMENT.         79 

gave  a  noticeable  increase  in  the  amount  of  growth  as  shown  by 
the -height  and  general  size  of  the  plant  as  the  result  of  hetero- 
zygosis. The  corolla  length  of  the  flowers,  which  is  very  little 
affected  by  environmental  factors,  was  not  increased  above  ttu 
average  of  the  two  parents. 

The  similarity  of  the  effects  of  heterozygosis  to  the  environ 
mental  effects  is  also  shown  in  the  affect  of  crossing  on  the  numbei 
of  nodes  and  internode  lengths  of  corn.  As  was  noted  fron 
Tables  15  and  13  the  number  of  nodes  is  increased  only  6  percen 
while  the  height  of  plant  is  increased  27  percent.  This  is  exactly 
the  effect  that  nutritional  factors  have.  The  height  of  plant  it 
reduced  under  poor  conditions  by  a  reduction  in  internode  length 
without  reducing  appreciably  the  number  of  nodes. 

In  general  it  is  evidently  true  that  heterozygosis  affects  many 
characters  in  the  same  way  as  the  environment,  but  it  should  be 
noted  that  in  time  of  maturity  these  two  factors  have  directly 
opposite  effects.  It  is  generally  recognized,  I  believe,  that  favor- 
able external  conditions  such  as  increased  moisture  or  fertility, 
where  these  are  limiting  factors,  which  result  in  a  greater  total 
amount  of  growth  tend  to  prolong  both  the  time  of  flowering  and 
the  completion  of  growth.  Conversely  unfavorable  external 
conditions  which  stunt  the  plants  and  limit  their  growth  tend  tc 
hasten  their  period  of  flowering  and  maturity.  There  are,  of 
course,  certain  exceptions  to  this  statement. 

Whether  or  not  the  effect  of  heterozygosis  in  hastening  maturity 
can  manifest  itself  independent  of  any  increase  in  vegetative 
luxuriance  or  other  manifestations  of  hybrid  vigor  is  not  known. 
The  results  given  here  would  indicate  that  the  vigor  derived  from 
crossing  enables  the  plant  to  carry  on  its  life  processes  more 
easily  and  more  efficiently  and  thus  to  accomplish  its  task  in  a 
shorter  time. 

With  regard  to  the  effects  of  heterozygosis  in  animals  much 
the  same  relation  is  shown  with  the  external  environmental 
effects  as  in  plants  although  the  rate  of  growth  and  size  obtained 
at  maturity  may  be  more  definitely  fixed  in  animals  than  in 
plants.  According  to  Castle  ('16)  there  is  an  increase  in  the  rate 
of  growth  as  well  as  the  attainment  of  a  larger  size  at  maturity 
in  hybrid  guinea-pigs.  Hyde  ('14)  also  finds  an  increase  in  rate 
of  growth  and  hastening  of  sexual  maturity  on  crossing  in  Droso- 
phila.    These  effects  in  animals  are  probably  greater  than  could 


80  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

be  obtained  by  any  amount  of  feeding  or  care  just  as  it  is  the  case 
in  plants. 

It  may  be  stated  briefly  that  the  effects  of  heterozygosis  in 
both  animals  and  plants,  not  too  distantly  related,  all  together 
contribute  towards  an  increased  reproductive  ability  and  this 
effect  has  probably  been  of  fundamental  importance  in  evolution 
in  establishing  sex. 


In  the  foregoing  account  of  some  of  the  most  noticeable  effects 
of  crossbreeding  upon  development  we  have  been  dealing  only 
with  crosses  among  closely  related  organisms.  It  is  of  course, 
well  known  that  in.  crosses  between  distantly  related  forms  the 
beneficial  effects  of  crossing  may  disappear  and  the  effects  become 
increasingly  more  injurious  as  the  degree  of  dissimilarity  becomes 
greater.  The  most  frequent,  pronouncedly  injurious  effect  is 
the  reduction  or  complete  loss  of  fertility.  This  may  or  may  not 
be  accompanied  by  a  great  acceleration  of  growth.  This  is 
shown  in  many  plants,  notably  by  Gravatt's  Radish-Cabbage 
hybrid  and  by  Wichura's  Willow  hybrids  as  well  as  bjr  many 
good  illustrations  given  by  Gartner  and  Focke.  It  is  perhaps 
not  surprising  that  the  reproductive  ability  should  be  the  first 
to  suffer  since  reproduction  is  the  most  difficult  task  the  organism 
has  to  perform.  The  failure  of  the  reproductive  mechanism 
might  divert  the  energies  into  bodily  growth  and  thus  in  part- 
account  for  the  large  size  and  great  vigor  of  some  sterile  hybrids 
but,  as  all  are  agreed,  this  can  not  entirely  account  for  the  great 
increases  in  size  nor  obviously  does  it  apply  to  the  more  common 
cases  where  both  size  and  productiveness  are  increased  at  the 
same  time. 

To  sum  up  one  can  therefore  say  that,  in  plants,  crossing  may 
have  a  great  range  of  effects,  according  to  the  degree  of  relation- 
ship of  the  parents,  from  a  condition  in  which:  the  cross  is  not 
possible  and  no  seed  produced;  seed  may  be  produced  but  fail 
to  germinate;  plants  may  be  produced  which  are  either  very  weak, 
normal  or  very  vigorous  without  being  able  to  reproduce  them- 
selves; plants  which  are  both  more  vigorous  and  more  productive 
than  their  parents;  to  a  condition  in  which  they  are  so  closely 
related  that  the  crossed  plants  do  not  differ  appreciably  from 
selfed  plants.     A  similar  series  can  be  arranged  with  animals. 


summary  of  the  ^effects.  81 

Summary  of  the  Effects  of  Inbreeding  and  Crossbreeding. 

Before  taking  up  a  theoretical  consideration  of  the  cause  of 
hybrid  vigor  and  its  importance  in  the  establishment  of  sex  it  is 
well  to  summarize  briefly  some  of  the  main  conclusions,  with 
regard  to  the  effects  of  inbreeding  and  crossbreeding  on  develop- 
ment, to  be  arrived  at  from  a  study  of  the  investigations  discussed. 

effects  of  inbreeding. 

1.  Continued  inbreeding  results  in  the  segregation  of  a  variable 
complex  into  a  number  of  diverse  types  which  are  uniform 
within  themselves. 

2.  The  segregates  which  differ  in  visible,  qualitative  characters 
also  differ  in  quantitative  characters;  types  with  abnormalities 
appear  which  cannot  reproduce  themselves;  others  appear  which 
are  perpetuated  with  difficulty;  others  are  obtained  which  are 
perfectly  normal  in  structure  and  function.  These  latter  are 
usually  less  well  developed,  but  may  be  as  well  or  better  developed 
than  the  original  stock  from  which  they  are  derived. 

3.  The  change  in  size,  structure,  or  function  and  reduction  in 
variability  is  most  noticeable  in  the  earlier  generations  of  in- 
breeding, rapidly  becomes  less  and  the  surviving  inbred  strains 
are  uniform  and  constant. 

4.  The  rate  of  approach  to  uniformity  and  constancy  differs 
in  different  lines. 

5.  These  uniform  and  constant  inbred  strains  are  quite  com- 
parable to  naturally  self-fertilized  species. 

6.  No  single  effect  can  be  attributed  to  inbreeding  other  than 
the  reduction  in  variability. 

7.  All  these  results  are  in  conformity  with  Mendel's  law  and 
Johannsen's  genotype  conception. 

THE   EFFECTS   OF    CROSSBREEDING. 

1.  Heterosis  accompanies  heterogeneity  in  germinal  constitu- 
tion whether  or  not  the  organisms  crossed  are  from  the  same  or 
diverse  stocks. 

2.  Heterosis  is  widespread  in  its  occurrence  throughout  the 
plant  and  animal  kingdoms. 

3* 


82  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

3.  Heterosis  is  shown  as  an  increase  in  the  size  of  parts  rather 
than  an  increase  in  the  number  of  parts. 

4.  Cross-fertilization  is  without  effect  until  the  zygote  is  formed; 
from  that  time  on  heterosis  may  be  apparent  throughout  the  life 
of  the  individual. 

5.  Heterozygosis  has  an  undiminished  effect  on  plants  propa- 
gated vegetatively. 

6.  Heterozygosis  may  have  a  stimulating  effect  on  some  char- 
acters and  a  depressing  effect  on  others  in  the  same  organism. 


A  Mendelian  Interpretation  of  Heterosis. 

It  is  due  to  the  work  of  G.  H.  Shull  ('08,  '09,  '10,  '11)  and  of 
East  ('08,  '09)  and  East  and  Hayes  ('12),  supplemented  and 
confirmed  by  the  results  given  here,  that  we  no  longer  believe 
that  inbreeding  is  a  process  of  continuous  degeneration.  Also 
these  investigators  first  demonstrated  clearly  that  the  same 
principle  was  involved  in  the  loss  of  vigor  accompanying  in- 
breeding and  the  increase  in  vigor  resulting  from  crossing. 

To  account  for  this  well  nigh  universal  loss  of  vigor  when  do- 
mesticated races  of  plants  and  animals  are  inbred,  they  thought 
it  necessary  to  assume  a  physiological  stimulation  which  was 
present  when  unlike  germplasms  were  united  and  which  disap- 
peared as  homozygosis  was  brought  about  automatically  by 
inbreeding.  Part  of  the  effects  of  inbreeding  were  due,  according 
to  their  views,  to  the  segregations  into  pure  lines  of  different 
hereditary  complexes  and  the  appearance  of  previously  hidden 
recessive  characters,  and  part  were  due  to  the  loss  of  this  stimu- 
lation. 

G.  H.  Shull's  ('14)  opinion  as  to  the  way  germinal  heterogeneity 
induces  vigor  is  stated  briefly  as  follows  (p.  126) : 

■"The  essential  features  of  the  hypothesis  may  be  stated  in  more  general 
terms  as  follows:  The  physiological  vigor  of  an  organism,  as  manifested 
in  its  "rapidity  of  growth,  its  height  and  general  robustness,  is  positively 
correlated  with  the  degree  of  dissimilarity  in  the  gametes  by  whose  union 
the  organism  has  been  formed.  In  other  words,  the  resultant  hetero- 
geneity and  lack  of  balance  produced  by  such  differences  in  the  reacting 
and  interacting  elements  of  the  germ-cells  act  as  a  stimulus  to  increased 
cell-division,  growth,  etc.  The  more  numerous  the  differences  between 
the  uniting  gametes — at  least  within  certain  limits — the  greater,  on  the 


A    MENDELIAN    INTERPRETATION    OF    HETEROSIS.  83 

whole,  is  the  amount  of  stimulation.  These  differences  need  not  be 
Mendelian  in  their  inheritance,  although  in  most  organisms  they  prob- 
ably "are  Mendelian  to  a  prevailing  extent." 

Both  the  view  stated  above  and  that  of  East  and  Hayes  assume 
that  the  increase  in  development  is  due  to  a  reaction  between 
different  elements  in  the  nucleus  and  that  this  stimulus  disappears 
when  homozygosity  is  reached.  A.  F.  Shull  ('12a)  has  proposed 
a  slightly  different  idea  in  that  he  assumes  the  stimulus  to  be  due 
to  the  reaction  of  new  elements  in  the  nucleus,  brought  in  by 
cross-fertilization,  to  the  maternal  cytoplasm.  According  to  his 
view  there  might  still  be  a  stimulation  even  after  complete  homo- 
zygosity is  attained.  Also  in  asexual  propagation  he  supposes 
that  the  cytoplasm  might  become  gradually  accustomed  to  a 
heterozygous  nucleus,  hence  long  continued  asexual  reproduction 
might  lead  to  a  gradual  reduction  in  vigor  which  this  writer  finds 
does  occur  in  the  rotifer,  Hydatina  senta.  ('12b). 

It  should  be  remembered,  however,  that  both  these  hypotheses, 
as  to  the  effect  of  germinal  differences,  postulate  a  stimulation  to 
account  for  an  increase  in  development  as  the  facts  demand.  It 
would  have  been  even  more  plausible  to  postulate  a  depressing 
effect  had  the  facts  been  otherwise.  The  only  basis  for  a  stimu- 
lation of  this  kind  is  in  the  fact  that  fertilization  initiates  the 
development  of  the  egg.  Heterozygosis,  however,  is  not  con- 
cerned with  the  starting  of  the  development  of  the  egg,  but  only 
with  the  rate  of  development  after  growth  is  commenced.  Is  it 
not  more  plausible  that  "a  lack  of  balance"  occasioned  by  the 
union  of  unlike  germplasms  would  retard  development  rather 
than  stimulate  it? 

Keeble  and  Pellew  ('10)  first  suggested  that  dominance  of 
characters  contributed  by  both  parents  might  be  a  factor  in  the 
increased  vigor  of  hybrids.  They  illustrated  this  conception  by 
a  cross  between  two  varieties  of  peas  which  possessed  features  of 
both  parents,  and  were  taller  than  either. 

Bruce  ('10)  has  shown  that  the  total  number  of  dominant 
factors  is  greater  in  a  hybrid  population  than  in  either  parental 
population  and  that  there  is  consequently  a  correlation  between 
the  number  of  dominant  factors  and  hybrid  vigor.  As  far  as  I 
know,  Bruce  has  never  followed  up  this  suggestion.  He  did  not 
show  why  it  was  that  the  presence  of  a  greater  number  of  domi- 
nant factors  brought  about  an  increase  in  growth,  nor  did  he 
13* 


84  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

show  why  it  was  that  all  the  dominant  features  could  rarely  or 
never  be  accumulated  in  certain  individuals  and  races  which  would 
therefore  show  no  reduction  in  vigor  when  inbred. 

East  and  Hayes  ('12)  attempting  to  distinguish  between  domi- 
nance and  the  effects  of  heterozygosis  make  the  following  state- 
ment (p.  31): 

"The  term  .vigor  has  hitherto  been  used  with  the  general  meaning 
which  the  biologist  readily  understands.  We  will  now  endeavor  to  show 
in  what  plant  characters  this  vigor  finds  expression.  It  is  not  an  easy 
task  because  of  the  possibility  of  confusing  the  phenomenon  of  Mendelian- 
dominance  with  the  physiological  effect  due  to  heterozygosis.  The  con- 
fusion is  due  to  a  superficial  resemblance  only.  Dominance  is  the  ex- 
pressed potency  of  a  character  in  a  cross  and  affects  the  character  as  a 
whole.  A  morphological  character,  like  the  pods  of  individual  maize 
seeds,  or  the  product  of  some  physiological  reaction  like  the  red  color  of 
the  seed  pericarp  in  maize,  may  be  perfectly  dominant,  that  is,  it  may  be 
developed  completely  when  obtained  from  only  one  parent.  Size  char- 
acters, on  the  other  hand,  usually  lack  dominance  or  at  least  show  in- 
complete dominance.  The  vigor  of  the  first  hybrid  generation  theoreti- 
cally has  nothing  to  do  with  these  facts.  This  is  easily  demonstrated  if 
one  remembers  that  the  increased  vigor  manifested  as  height  in  the  Fi 
generation  cannot  be  obtained  as  a  pure  homozygous  Mendelian  segregate, 
which  would  be  possible  if  due  to  dominance.  Furthermore,  the  univer- 
sality with  which  vigor  of  heterozygosis  is  expressed  as  height  shows  the 
distinction  between  the  two  phenomena.  If  the  greater  height  were  the 
expression  of  the  meeting  of  two  factors  (T2,  t2,  x  ti,  T2)  both  of  which  were 
necessary  to  produce  the  character,  one  could  not  account  for  the  frequency 
of  the  occurrence.  Nevertheless,  in  practice  the  confusion  exists,  and 
while  we  have  considerable  confidence  in  the  conclusions  drawn  from  our 
experiments,  we  have  no  intention  of  expressing  them  dogmatically.". 

G.  H.  ShulPs  statements  of  the  way  in  which  crossing  brings 
about  increased  development,  and  the  relation  that  this  stimula- 
tion of  growth  has  to  dominance  of  Mendelian  characters  is  fairly 
stated,  I  believe,  in  the  following  passage  ('11,  pp.  244-245): 

"In  1908  I  suggested  a  hypothesis  to  explain  the  apparent  deterioration 
attendant  upon  self-fertilization,  by  pointing  out  that  in  plants,  such  as 
maize,  which  show  superiority  as  a  result  of  cross-fertilization,  this 
superiority  is  of  the  same  nature  as  that  so  generally  met  with  in  Fi 
hybrids.  I  assumed  that  the  vigor  in  such  cases  is  due  to  the  presence 
of  heterozygous  elements  in  the  hybrids,  and  that  the  degree  of  vigor  is 
correlated  with  the  number  of  characters  in  respect  to  which  the  hybrids 
are  heterozygous.  I  do  not  believe  that  this  correlation  is  perfect,  of 
course,  but  approximate,  as  it  is  readily  conceivable  that  even  though 
the  general  principle  should  be  correct,  heterozygosis  in  some  elements 


A   MENDELIAN   INTERPRETATION   OF   HETEROSIS.  85 

may  be  without  effect  upon  vigor,  or  even  depressing.  The  presence  of 
unpaired  genes,  or  the  presence  of  unlike  or  unequal  paired  genes,  was 
assumed  to  produce  the  greater  functional  activity  upon  which  larger 
size  and  greater  efficiency  depend.  This  idea  has  been  elaborated  by 
Dr.  East  and  shown  to  agree  with  his  own  extensive  experiments  in  self- 
fertilizing  and  crossing  maize.  He  suggests  that  this  stimulation  due  to 
hybridity  may  be  analogous  to  that  of  ionization. 

Mr.  A.  B.  Bruce  proposes  a  slightly  different  hypothesis  in  which  the 
degree  of  vigor  is  assumed  to  depend  upon  the  number  of  dominant 
elements  present,  rather  than  the  number  of  heterozygous  elements. 
While  all  of  my  data  thus  far  are  in  perfect  accord  with  my  own  hypothe- 
sis, and  I  know  of  no  instance  in  which  self-fertilization  of  a  corn-plant 
of  maximum  vigor  has  not  resulted  in  a  less  vigorous  progeny,  it  is  quite 
possible  that  I  have  still  insufficient  data  from  which  to  distinguish 
between  the  results  expected  under  these  two  hypotheses.  However, 
for  the  purpose  of  the  present  discussion,  it  is  not  necessary  to  decide 
which  of  these  two  hypotheses  (if  either)  is  correct.  Both  of  them  are 
based  upon  the  view  that  the  germ-cells  produced  by  any  plant  whose 
vigor  has  been  increased  by  crossing  are  not  uniform,  some  possessing 
positive  elements  or  genes  not  possessed  by  others." 

A.  F.  Shull  does  not  consider  dominance  as  an  adequate  means 
of  accounting  for  heterosis,  agreeing  with  East  and  Hayes  and 
G.  H.  Shull,  as  the  following  quotation  shows:    ('12a,  p.  10) 

"The  view  that  vigor  depends  upon  heterozygosis  of  the  individual 
seems  to  me  inherently  more  probable  than  that  it  is  due  to  the  presence 
of  certain  dominant  genes.  The  former  view  admits  of  a  plausible  foun- 
dation in  cell  physiology,  and  the  essence  of  it  may  be  extended  to  cases 
of  decrease  of  vigor  in  which  there  is  no  change  in  genotypic  constitution, 
and  which  are  therefore  without  the  pale  of  either  theory." 

Castle  is  also  in  accord  with  the  general  belief  that  heterosis  is 
not  due  to  dominance  of  factors  and  draws  a  distinction  between 
the  effects  of  inherited  characters  and  the  stimulus  resulting  from 
crossing.  In  speaking  of  the  increase'  in  size  in  crosses  between 
diverse  races  of  guinea-pigs  he  says:    ('16,  p.  212.) 

"So  far  as  heredity  is  concerned,  the  inheritance  is  blending,  but  Fi 
shows  an  increase  in  size  due  to  hybridization.  This  increased  size, 
however,  does  not  persist  into  F2.  It  seems  to  be  due  not  to  heredity  at  all." 

(And  again  on  pages  223  and  224.) 

"Cross  breeding  has,  then,  the  same  advantage  over  close  breeding  that 
fertilization  has  over  parthenogenesis.  It  brings  together  differentiating 
gametes,  which,  reacting  on  each  other,  produce  greater  metabolic  activity. 
Whether  or  not  the  uniting  gametes  differ  by  Mendelian  unit-characters 


86  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

is  probably  of  no  consequence.  That  they  differ  chemically  is  doubtless 
the  essential  thing  in  producing  added  vigor.  Heterozygosis  is  mentioned 
merely  as  an  evidence  of  such  chemical  difference." 

These  quotations  suffice  to  show  that  a  distinction  is  held  by- 
biologists  at  the  present  time  between  the  effects  of  inbreeding 
and  cross  breeding  and  of  heredity  in  development,  and  they 
believe  that  dominance  of  hereditary  factors  is  inadequate  to 
account  for  the  widespread,  if  not  universal  phenomenon  of 
heterosis.  The  reason  why  biologists  in  general  have  refused  to 
believe  that  dominance  was  in  any  way  responsible  for  the  in- 
creased vigor  of  hybrids  has  been  due  to  two  objections  which 
have  seemed  to  make  this  hypothesis  untenable.  They  thought 
that  if  hybrid  vigor  was  due  to  the  dominance  of  definitely  in- 
herited characters  that  all  these  favorable  characters  which  bring 
about  heterosis  could  be  easily  recombined  into  a  homozygous 
individual  which  would  show  no  reduction  on  subsequent  inbreed- 
ing. Since  no  clear  case  was  known  in  maize  where  a  plant  did 
not  lose  vigor  on  inbreeding  this  seemed  to  be  a  convincing  argu- 
ment. Another  objection  to  dominance  as  a  means  of  accounting 
for  heterosis  was  raised  by  Emerson  and  East  ('13)  in  that  the 
distribution  in  F2  should  be  unsymmetrical  in  respect  to  those 
characters  in  which  heterosis  was  shown  in  Fx.  Since  the  usual 
frequency  distributions  in  cases  of  this  kind  are  symmetrical,  this 
objection  appeared  to  be  valid. 

How  both  of  these  objections  do  not  hold  when  linkage  of  hered- 
ity factors  is  taken  into  consideration,  the  writer  has  attempted 
to  show  in  a  recent  publication  ('17).  Because  of  linkage,  char- 
acters tend  to  pass  from  one  generation  to  the  next  in  groups  and 
are  not  easily  recombined.  Furthermore,  on  account  of  linkage 
skewness  is  not  expected  in  the  second  hybrid  generation.  All  of 
the  recently  acquired  knowledge  of  heredity  makes  it  seem  highly 
probable  that  heterosis  may  be  largely,  if  not  entirely,  accountable 
on  the  basis  of  dominance  of  linked  factors. 

In  considering  these  two  hypotheses,  both  attempting  to  ac- 
count for  heterosis,  the  following  facts  about  dominance  should 
be  kept  in  mind: 

1.  Partial  dominance  of  characters  is  a  widespread  occurrence 
in  plants  and  animals. 

2.  Dominance,  of  course,  does  not  appear  until  after  the  zygote 
is  formed: 


A    MENDELIAN    INTERPRETATION    OF    HETEROSIS.  87 

3.  In  most  cases  dominance  does  not  change  throughout  the 
life  of  the  individual  and  remains  the  same  through  innumerable 
clonal  generations. 

While  none  of  these  features  of  dominance  offers  any  definite 
means  of  proving  the  truth  of  the  hypothesis  advanced,  is  it  only 
a  coincidence  that  they  fit  in  exactly  with  what  the  facts  of  het- 
erosis demand?  It  remains  to  show  that  those  characters  which 
enable  a  plant  or  animal  to  obtain  the  best  development  are,  for 
the  most  part  at  least,  partially  dominant  over  those  characters 
which  retard  or  prevent  maximum  growth. 

The  essential  difference  between  the  two  hypotheses  may  be 
stated  briefly.  According  to  the  previous  view  the  hybrid  combi- 
nation of  factors  Aa  carried  the  ability  to  stimulate  development 
because  of  the  union  of.  unlike  elements.  This  stimulation  was 
absent  in  either  of  the  homozygous  combinations  AA  and  aa, 
and  this  stimulation  had  no  direct  relation  to  the  part  that  either 
A  or  a  had  in  development  as  hereditary  entities.  According  to 
the  conception  of  dominance,  first  proposed  by  Keeble  and 
Pellew  and  carried  out  more  fully  by  the  writer,  the  hybrid  union 
of  AAbb  with  aaBB,  resulting  in  the  heterozygous  combination 
of  Aa  Bb,  increases  development  because  two  dominant  characters 
are  present  here  together,  whereas  each  parent  has  only  one 
dominant  character.  A  similar  factorial  arrangement  has  been 
proposed  by  Hyde  ('14)  to  account  for  the  increased  fertility  of 
his  crosses  among  partially  sterile  strains  of  Drosophila. 

In  crosses  between  different  types  of  domesticated  animals  and 
of  cultivated  plants  it  has  frequently  been  noted  that  there  is  a 
tendency  towards  a  return  to  the  characters  of  the  wild  species 
from  which  they  were  derived.  Sageret  ('26)  makes  particular 
note  of  this  point.  It  is  well  known  that  crosses  between  different 
breeds  of  pigeons  is  quite  apt  to  bring  back  the  wild-type  of 
plumage.  The  hybrid  between  radish  and  cabbage  described  by 
Gravatt  ('14)  illustrates  this  point  strikingly.  The  hybrid  pro- 
duced had  neither  a  succulent  "head"  like  its  cultivated  male 
parent  nor  a  fleshy  root  like  its  female  parent.  In  other  respects, 
as  well,  it  showed  this  return  to  wild-type  characters.  It  was 
also  exceedingly  vigorous,  but  sterile,  like  so  many  hybrids  between 
diverse  stocks. 

Drosophila  furnishes  the  best  illustration  of  the  appearance  of 
wild  type  characters  in  the  first  hybrid  generation.    Of  the  more 


88  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

than  one  hundred  mutations  found  in  Drosophila  by  far  the 
largest  number  of  these  are  recessive.  Almost  all  of  them  are 
characters  which  are  less  favorable  to  development.  It  is  stated 
that  any  attempt  to  collect  a  large  number  of  the  recessive  char- 
acters into  one  race  is  rendered  difficult  by  the  weakened  consti- 
tution of  the  flies  possessing  any  great  accumulation  of  .recessive 
characters  (Muller,  '16).  Whenever  crosses  are  made  between 
diverse  types  the  first  generation  fly  is  in  many  of  its  characters 
more  like  the  wildstock  and  hence  more  vigorous  than  its  parents. 
All  lethal  factors,  well  illustrated  in  Drosophila,  furnish  additional 
support  to  the  hypothesis  of  dominance  as  a  means  of  accounting 
for  heterosis.  Muller  ('17)  has  shown  that  a  condition  of  "bal- 
anced lethals"  may  be  brought  about  in  which  only  the  hetero- 
zygotes  can  live.  As  dominant  lethal  factors  are  always  eliminated 
as  soon  as  they  occur,  so,  also,  is  there  always  a  strong  tendency 
for  selection  to  eliminate  any  dominant  character  which  is  at  all 
unfavorable  to  the  organism's  best  development.  Unfavorable 
recessive  factors  also  tend  to  be  eliminated,  but  much  more  slowly. 

If  the  results  obtained  in  Drosophila  are  applicable  to  other 
animals  and  to  plants  we  must  infer  that  recessive  mutations 
occur  the  most  commonly.  Hence  recessive  mutations  make  up 
the  characters,  to  a  large  degree,  that  man  has  selected  in  the 
production  of  domesticated  animals  and  plants.  Just  as  in  Dro- 
sophila, crosses  between  diverse  domesticated  types  tend  to 
result  in  the  reappearance  of  wild-type  characters  which  are  more 
useful  to  the  plant  or  animal  whose  chief  aim  in  life  is,  apparently, 
to  reproduce  itself. 

This  is  well  shown  in  an  illustration  from  maize.  As  stated 
before,  inbred  strains  have  been  obtained  which  are  markedly 
deficient  in  root  development.  On  these  plants  the  large  brace 
roots  which  commonly  appear  when  the  plants  begin  to  need  extra 
support,  are  almost  completely  lacking.  Consequently,  the  plants 
are  blown  over  when  they  become  heavy  at  the  time  of  ear  for- 
mation. I  have  observed  these  strains  three  years  and  each  time 
they  have  fallen  down.  This  character  is  not  determined  by  soil 
conditions  or  insect  damage  or  any  external  conditions  as  far  as 
can  be  seen.  Other  plants  on  either  side  are  perfectly  upright. 
When  these  strains  are  crossed  with  other  strains,  inbred  for  an 
equal  or  longer  period,  which  have  well  developed  brace  roots, 
the  first  hybrid  generation  has  remarkably  well  developed  brace 


A   MENDELIAN   INTERPRETATION    OF   HETEROSIS.  89 

roots,  and  usually  does  not  show  the  slightest  tendency  to  go  down, 
as  shown  in  Plates  Xlla  and  b.  Emerson  (712)  describes  similar 
plants  in  which  the  root  deficiency  is  also  recessive.  Another 
striking  feature  is  shown  in  this  illustration.  The  inbred  strain 
which  lacks  brace  roots  is  derived  from  a  floury  variety  of  corn 
and  shows  a  decided  tendency  to  branch  at  the  base  of  the  stalk. 
These  branches  form  stalks  with  tassels  and  ears  and  many  of 
them  are  fully  as  well  developed  as  the  main  stalk.  .  In  this  way 
two  or  three  stalks  may  be  developed  from  one  seed.  The  other 
parent  of  the  cross  shown  never  branches  in  this  way  and  never 
even  develops  small  branches  or  "suckers."  The  first  hybrid 
generation  shows  this  tendency  to  branch  even  more  strongly 
developed  than  the  branching  parent.  The  plants  shown  are  from 
three  hills  grown  side  by  side  and  each  hill  is  the  product  of  three 
seeds.  Thus  it  will  be  seen  that  both  parents  have  contributed 
characters  to  the  hybrid.  Both  these  characters  are  such  as  to 
enable  the  plants  to  attain  a  greater  development  in  general 
vegetative  luxuriance  than  would  be  possible  if  either  were  lacking. 
Emerson  ('12)  gives  an  even  better  illustration  of  two  extremely 
unproductive  types  of  maize  which  give  a  vigorous  hybrid,  one 
of  the  parents  contributing  tall  stature,  the  other  green  chlorophyll. 

Many  more  illustrations  of  a  similar  operation  of  hereditary 
factors  favoring  a  hybrid  in  its  development  might  be  cited.  I 
believe  that  enough  have  been  given  to  clear  the  way  towards  the 
acceptance  of  the  doctrine  that  hybrid  vigor  is  due  largely  to  the 
normal  functioning  of  definable,  hereditary  factors. 

It  is  recognized  that  the  characters  used  as  illustrations  here  are 
superficial  in  nature.  The  characters  which  are  really  concerned 
in  heterosis  are  those  deep-seated,  fundamental,  physiological 
processes  which  govern  metabolism  and  cell-division.  As  to  the 
mode  of  inheritance  of  these  characters  we,  as  yet,  know  little. 
There  is  no  reason  to  believe,  however,  but  that  many  or  all  of 
them  are  Mendelian  in  mode  of  inheritance  and  that  many  of 
them  operate  in  the  same  way  to  enable  hybrid  progeny  to 
attain  a  more  complete  development  than  their  parents.  If  this 
hypothesis,  as  to  the  way  in  which  heterosis  is  brought  about,  is 
in  its  essential  features  correct,  it  points  the  way  towards  a  more 
fundamental  application  of  Mendelism  to  the  physiological 
processes  of  growth  than  is  generally  acceded. 


90  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

There  now  remains  to  be  discussed  the  part  that  crossing-over 
or  breaks  in  the  linkage  of  hereditary  factors  would  play  according 
to  this  hypothesis.  If  any  large  number  of  characters  are  con- 
cerned and  the  dominant  and  recessive  genes  are  equally  appor- 
tioned between  the  two  parents,  and  distributed  at  random  on 
the  chromosomes,  the  chance  of  crossovers  occurring  in  such  a 
way  as  to  bring  all  the  dominant  factors  in  one  individual  at  one 
time  would  be  almost  inconceivably  small,  especially  when  the 
phenomenon  of  interference  is  taken  into  consideration.  How- 
ever, when  crossing-over  does  occur  in  such  a  way  as  to  bring 
about  more  fortunate  combinations  in  certain  individuals,  those 
individuals  would  be  the  ones  selected  by  man  in  domesticated 
races,  or  by  nature  in  the  wild.  Partial  linkage  does  not  prevent 
recombination  but  merely  adds  to  the  complexity  of  the  process. 
The  chance  of  fortunate  recombinations  would  be  greater  in  the 
more  widely  crossed  animals  and  plants  but  such  combinations 
would  be  again  broken  up  by  further  crossing.  The  tendency 
would  be,  however,  for  the  best  combination  of  characters  to 
survive  and  gradually  supplant  the  others  in  time.  In  naturally 
selfed  plants,  most  of  which  are  crossed  at  more  or  less  infrequent 
intervals,  a  fortunate  homozygous  combination  would  be  fixed 
and  the  plants  possessing  such  combinations  would  in  time  sup- 
plant their  less  fortunate  relations. 

Thus  there  would  always  be  the  tendency  for  all  the  more 
favorable  characters  to  be  gathered  together  and  the  others 
eliminated.  In  time  all  the  individuals  of  a  locality  would  tend 
to  become  equal  in  their  hereditary  characters  and  crossing 
between  individuals  in  a  given  locality  would  not  accumulate 
any  greater  number  of  favorable  characters  than  the  parents 
possessed  and  hence  would  not  show  any  evidences  of  heterosis. 

That  this  is  the  condition  which  is  brought  about  Darwin  has 
shown.  Individuals  from  the  same  locality  derive  little  or  no 
benefit  from  crossing  while  crosses  between  individuals  from 
different  geographical  regions  show  a  greater  effect  of  crossing. 
The  work  of  Collins  ('10)  and  the  results  obtained  at  the  Con- 
necticut Station  (Jones  and  Hayes  '17)  show  this  also — varieties 
of  maize  of  similar  characters  and  from  the  same  region  give  less 
increase  when  crossed  than  do  varieties  of  diverse  type  or  from 
widely  separated  geographical  regions. 


A   MENDELIAN   INTERPRETATION    OF   HETEROSIS.  91 

If  by  Crossing-over  and  subsequent  recombination  the  charac- 
ters which  bring  about  the  great  development  in  Fi  can  all  be 
accumulated  in  a  homozygous  condition  in  an  individual,  that 
individual  should  show  a  greater  development  even  than  the  Fi 
as  A.  F.  Shull  ('12)  has  pointed  out.  This  is  on  the  assumption 
that  most  characters  which  play  a  part  in  heterosis  are  not  fully 
dominant.  That  a  factor  in  the  diploid  condition  has  a  greater 
effect  than  when  in  the  haploid  condition  is  indicated  by  the 
work  of  Hayes  and  East  ('15)  on  endosperm  characters  in  maize. 
Their  results  show  that  in  reciprocal  crosses  a  double  dose  of 
one  allelomorph  in  the  maternal  endosperm  fusion  nucleus  over- 
comes a  single  dose  in  the  paternal  endosperm  nucleus.  In  other 
words  factors  have  an  accumulative  effect. 

The  evidence  that  such  superior  individuals  have  been  obtained 
by  inbreeding  is  not  very  convincing  it  must  be  admitted.  Dar- 
win, however,  in  Ipomea  obtained  plants- — "  Hero  "  and  its 
descendants— which  were  certainly  no  less  vigorous  than  any 
plants  at  the  beginning  of  the  inbreeding  period  and  the  same 
thing  occurred  in  Mimulus.  These  are  the  two  species  which 
were  the  most  extensively  inbred.  Miss  King,  as  mentioned 
before,  has  obtained  inbred  rats  which  are  larger  and  more 
vigorous  than  individuals  present  in  the  original  stock.  Nothing 
of  this  kind  has  occurred  in  maize  and  on  account  of  the  small 
chance  of  recombining  many  of  the  most  desirable  characters 
in  one  plant,  it  is  not  at  all  surprising  that  such  individuals  have 
not  as  yet  been  produced. 

The  production  of  individuals  by  inbreeding  which  excel  any 
of  the  original  crossbred  stock  offers  some  means  of  deciding 
between  the  two  hypothesis  attempting  to  account  for  heterosis. 
According  to  the  hypothesis  of  a  physiological  stimulation  it 
would  be  difficult  to  see  how  individuals  more  vigorous  than 
the  parents  could  be  produced  by  inbreeding. 

The  hypothesis  of  dominance  also,  possibly,  makes  it  easier  to 
understand  why  naturally  crossed  wild  species,  which  have  not 
been  outcrossed  with  fresh  stocks  for  long  periods  of  time,  may 
not  show  any  markedly  injurious  effects  from  artificial  inbreeding. 
According  to  the  former  view  different  characters  of  equal  value 
to  the  organism  which  might  persist  indefinitely  in  a  species, 
would  supply  a  stimulation  when  united  in  a  heterozygous  com- 
bination.     This  stimulation  would  be  lost  whenever  individuals 


92  CONNECTICUT   EXPERIMENT   STATION   BULLETIN   207. 

were  reduced  to  homozygosity  by  artificial  self-fertilization. 
According  to  the  view  of  dominance  if  the  allelomorphs  were  all 
equal  in  their  contributions  to  development  there  might  be 
differences  in  a  species  and  still  no  loss  of"  vigor  would  result 
from  inbreeding.  It  is  assumed  that  the  less  favorable  characters 
have  been  eliminated  by  selection.  On  either  hypothesis  there 
would  be  no  reduction  from  inbreeding  if  all  the  members  of 
species  were  exactly  alike  whether  they  are  naturally  crossed 
or  naturally  self-fertilized. 

The  hypothesis  of  physiological  stimulation  also  implies  the 
assumption  that  naturally  crossed  species  of  cultivated  plants 
are  inherently  more  efficient  as  producers  than  naturally  selfed 
plants.  This  is  hardly  justified  when  we  recall  such  vigorous 
and  productive  plants  as  wheat,  oats,  barley,  rice,  peas,  beans, 
tobacco,  tomatoes  and  many  others  which  are  usually  self  pol- 
linated. It  is,  however,  difficult  to  make  a  fair  comparison  on 
this  basis. 

To  sum  up,  it  may  then  be  stated  briefly  that  dominance  of 
characters  as  opposed  to  the  former  idea  of  an  indefinable  physio- 
logical stimulation  makes  more  understandable  the  facts  that : 

1.  Heterozygosis  produces  a  stimulating,  and  not  an  indifferent 
or  depressing  effect  in  crosses  between  related  stocks  and  that 
the  reverse  is  true  in  widely  diverse  stocks. 

2.  Heterozygosis  operates  throughout  the  lifetime  of  the 
individual  even  through  many  generations  of  vegetative  propa- 
gation. 

3.  Inbreeding  may  result  in  individuals  more  vigorous  than  the 
original  cross-bred  stock. 

4.  Inbreeding  may  not  bring  about  a  reduction  in  some  naturally 
crossed  wild  species. 

Whether  or  not  dominance  of .  factors  is  wholly  adequate  to 
account  for  all  of  the  immediate  effects  of  exogamy  remains  to 
be  seen.  The  former  view  that  dominance  was  not  concerned 
at  all  has  been  maintained  so  insistently  that  I  have  taken  the 
extremely  opposite  view  in  order  to  show  that  dominance  at  least 
can  be  held  responsible  for  a  large  part  of  the  increased  develop- 
ment shown  by  hybrids.  The  treatment  of  the  subject  in  this 
light  has  been  dogmatic.  That  cross-fertilization  may  produce 
some  effect  which  can  never  be  attained  in  self-fertilization  or  a 
sexual  reproduction  is  still  possible.     The  view  of  the  problem 


HETEROSIS    AND   THE    ESTABLISHMENT    OF   SEX.  93 

which  is  presented  here  makes  certain  heretofore  indefinite  effects 
more  intelligible.  It  is  not  meant  to  preclude  entirely  any  bene- 
ficial physiological  stimulation  resulting  from  germinal  diversity, 
if  such  an  effect  can  be  demonstrated. 

The  difference  between  the  two  hypotheses  are  not  as  great  as 
might  seem  at  first  sight.  The  older  hypothesis  is  general  in  its 
application  and  does  not  commit  itself  to  the  interpretation  of 
specific  effects.  The  view  presented  here  is  specific  in  its  applica- 
tion and  may  be  shown  to  be  inadequate  for  the  interpretation 
of  all  phases  of  the  problem  of  increased  development  following 
cross-fertilization . 

The  greatest  progress  in  our  knowledge  of  inbreeding  and  cross- 
breeding was  made  when  their  effects  were  linked  with  Mendelian 
phenomena.  This  was  the  big  step  forward.  The  two  ways  of 
interpreting  these  effects  discussed  here,  differ  only  in  minor 
features  and  it  is  not  putting  the  matter  fairly  to  hold  them  up  as 
two  rival  hypotheses,  one  to  be  chosen  from  the  other.  Placing 
the  effects  of  inbreeding  and  cross-breeding  entirely  on  a  Mendelian 
basis  is  merely  the  logical  outgrowth  of  the  older  view  as  knowledge 
of  the  methods  of  inheritance  increased. 


THE    PART   THAT    HETEROSIS    HAS    PLAYED    IN    THE    ESTABLISHMENT 

OF   SEX. 

Since  heterosis  is  widespread  in  its  manifestation  it  can  hardly 
be  doubted  that  it  has  played  some  part  in  the  initiation  and 
maintenance  of  sexual  differentiation  in  organisms.  Jennings 
('13),  however,  has  shown  that  conjugation  in  Paramecium  does 
not  result,  immediately,  to  the  advantage  of  the  organism.  The 
rate  of  reproduction  is  actually  diminished  and  many  of  the 
organisms  perish.  The  advantage  which  is  derived  from  con- 
jugation, he  considers  with  Weismann,  is  due  to  the  fact  that 
biparental  inheritance  makes  possible  a  greater  variability  and 
consequently  a  greater  chance  of  recombinations,  some  of  which 
are  better  able  to  persist.  Hence,  while  many  offspring  from 
conjugating  paramecia  die,  some  may  be  able  to  survive. 
Conjugation  therefore  makes  possible  a  greater  elasticity  in 
adaptiveness  to  new  and  varied  surroundings. 

If  this  immediately  depressing  effect  found  in  Paramecium  is 
general  in  the  lower  animals,   heterosis  would   probably  have 


94  CONNECTICUT   EXPERIMENT   STATION   BULLETIN    207. 

played  no  part  in  the  inauguration  of  sex.  Both  A.  F.  Shull 
('12a)  and  Whitney  ('12a)  have  shown,  however,  that  heterosis 
occurs  in  the  rotifer,  Hydatina  senta. 

In  the  lower  plants  heterosis  would  have  significance  only  in 
spore  formation,,  as  the  main  life  of  the  plant  is  carried  on  in  the 
haploid  condition  where  heterozygosis  could  not,  of  course, 
operate.  As  organisms  became  more  differentiated  and  specialized 
the  accumulation  of  factors  in  the  zygote  from  two  somewhat 
different  parents  would  have  increasing  significance.  If,  for 
example,  an  organism  should  vary  in  a  character  A  by  one  new 
dominant  mutation  A',  the  heterozygote  AA',  according  to  the 
hypothesis  of  dominance,  would  be  superior  to  the  combination 
AA  but  not  to  the  combination  A'A'.  According  to  the  former 
view  of  a  physiological  stimulation  the  heterozygous  combination 
AA'  might  be  superior  to  either  homozygous  combination.  The 
matter  is  not  so  simple  as  this,  however.  The  breeding  facts 
show  that  recessive  unfavorable  variations  are  far  more  common 
than  dominant  favorable  ones.  The  chances  would  be  that 
those  individuals  which  varied  by  dominant  mutations  would 
also  vary  from  the  parental  stock,  sooner  or  later,  by  recessive 
mutations  as  well,  so  that  any  hybrid  union  would  tend  to  accu- 
mulate more  favorable  factors  than  either  parental  individual 
possessed  and  hence  show  heterosis.  Heterosis  would  be  an 
immedate  factor  for  natural  selection  to  work  upon. 

Moreover  it  seems  possible  that  heterosis  has  had  considerable 
to  do  with  the  rise  of  the  sporophyte  and  the  decline  of  the  game- 
tophyte  in  plants.  Recombination  of  characters  can  take  place 
as  well  when  the  dominant  generation  is  the  haploid  as  well  as 
when  it  is  diploid  in  respect  to  the  chromosome  arrangement. 
From  the  standpoint  of  adaptiveness  through  recombination 
of  characters  it  might  even  be  to  the  organism's  advantage  to 
retain  the  haploid  generation  as  the  one  in  which  the  principal 
life  processes  were  carried  on,  since  the  different  combinations 
would  then  be  more  surely  tested  and  the  best  more  easily 
selected  in  the  simplex  than  in  the  duplex  condition.  Heterosis 
can  only  operate  in  the  sporophyte.  The  union  of  different 
hereditary  complexes  gives  to  the  sporophyte  an  advantage  over 
the  gametophyte  in  that  all  new  favorable  variations  work  to- 
gether whereas  segregation  in  the  formation  of  the  gametophyte 
reduces  the  efficiency  of  this  generation.     On  the  basis  of  the 


HETEROSIS  AND   THE    ESTABLISHMENT    OF    SEX.  95 

complimentary  action  of  factors  according  to  the  dominance 
hypothesis  of  heterosis  the  gametophyte  would  practically  always 
be  at  a  disadvantage  as  compared  to  the  sporophyt'e  as  long  as 
variations  were  occurring  so  that  heterosis  must  have  played 
some  part  in  these  important  changes. 

Either  on  the  basis  of  inducing  variability  or  stimulating 
development,  sex  would  be  a  creation  of  no  value  to  organisms 
which  are  never  cross-fertilized.  It  may  be  questioned  if  many 
such  exist.  In  either  case  the  sexual  mechanism  is  so  complex 
and  deep-seated  in  the  life  of  the  organism  that  it  is  not  to  be 
discarded  easily.  Whenever  the  best  possible  combination  of 
factors  for  a  given  environment  is  produced,  it  is  to  the  advantage 
of  the  organism  possessing  that  combination  to  give  up  cross- 
fertilization  and  resort  to  either  self-fertilization  or  some  form 
of  sexual  reproduction,  for  the  reason  that  these  are  more  efficient 
means  of  propagation.  When  the  environment  changes,  those 
organisms  which  are  not  cross-fertilized  may  either  be  doomed 
to  extinction  or  handicapped  in  becoming  adapted  to  new  con- 
ditions and  the  perpetuation  of  the  sexual  mechanism  thereby 
accounted  for. 

Whatever  may  be  the  value  or  significance  of  heterosis,  to 
account  for  this  phenomenon  it  is,  for  the  most  part,  unnecessary 
to  assume  that  there  is  an  indefinite  stimulating  effect  of  hybrid- 
ization along  with  the  expression  of  definable  hereditary  factors. 
Hence  the  distinction  is  no  longer  needed  between  the  effects  of 
self-ferti  ization  and  cross-fertilization  and  of  heredity  in  develop- 
ment. The  heretofore  indefinite  physiological  stimulation  re- 
sulting from  heterozygosis  and  the  related  effects  accompanying 
the  loss  of  this  stimulation  following  inbreeding  can  therefore 
be  given  a  strictly  Mendelian  interpretation. 

This  being  so  there  is  no  longer  a  question  as  to  whether  or  not 
inbreeding  per  se  is  injurious.  Whether  good  or  bad  results  from 
inbreeding  depends  solely  on  the  constitution  of  the  organisms 
before  inbreeding  is  commenced.  Inbreeding  is  concerned  only 
with  the  manifestation  of  conditions  pre-existing.  As  a  means  of 
analyzing  and  of  purifying  a  cross-bred  stock  by  the  elimination 
of  undesirable  qualities,  inbreeding  is  therefore  a  method  of  first 
importance   in   plant   and   animal   improvement. 


96  CONNECTICUT   EXPERIMENT   STATION    BULLETIN   207. 

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Agr.  7:    265-272. 


PLATE  I. 


a.     A  non-inbred  variety  of  Learning  dent  corn. 


^.g»< 


b.     Four  inbred  strains  derived  from  the  Learning  variety  after  nine  gen- 
erations of  self-fertilization  showing  an  ear,  a  cob  and 
a  cross-section  of  a  cob  of  each. 
4 


PLATE  II. 


a.     Representative  ears  of  inbred  strain  No.  1-6-1-3,  etc. 


b.     Representative  ears  of  inbred  strain  No.  1-9-1-2,   etc.,   above  and  No. 

1-7-1-1,  etc.,  below. 


PLATE  III. 


a     Representative  ears  of  inbred  strain  No.  1-7-1-2,  etc. 


b.     The  first  generation  cro§s  of  inbred  strain  No.   1-6-1-3  by  1-7-1-2. 


(Plates  I  to  III  inclusive,  with  the  exception  of  lb,  are  on  the  same  scale).  The  plants 
which  produced  these  ears  were  all  grown  oh  the  same  field  and  the  non-inbred  variety  and 
the  first  generation  cross  were  grown  in  adjoining  iows.  The  ears  of  these  latter  two  represent 
the  best  ears  produced  by  60  plants  of  each;  !■  J.u| yli' 

14* 


PLATE  IV. 


m'i 


mMmmSM 


Representative   plants  of  the   original,   non-inbred   Learning  variety. 


b.     Representative  plants  of  the  inbred  strain  No.  1-6-1-3,  etc. 


PLATE  V. 


Representative  plants  of  the  inbred  strain  No.  1-7-1-2,  etc. 


b      Representative  plants  of  the  first  generation  cross   of   inbred   strain  No. 

1-6-1-3  by  1-7-1-2. 

(Plates  IV  and  V  are  on  the  same  scale.) 


PLATE  VI. 


a.  Two  fully  developed  tassels  on  the  left  and  two  partially  sterile  tassels  on 
the  right  characteristic  of  four  different  inbred  strains  of  maize.  From  left  to 
right  they  are,  20A-8-5-10;    1-9-1-2;    1-6-1-3;    21-3-13-9. 


b.  Representative  ears  from  the  corresponding  strains  shown  in  the  illus- 
tration above.  The  first  strain  on  the  left  produces  fully  developed  tassels 
and  moderately  developed  ears.  The  second  produces  the  best  developed 
tassels  and  the  poorest  ears.  The  other  two  have  poorly  developed  tassels 
and   moderately   well   developed   ears. 


PLATE  VII. 


a.  Two  inbred  strains  of  dent  corn,  No.  1-6-1-3  at  the  right  and  No. 
1-7-1-1  at  the  left  and  the  first  generation  cross  in  the  center.  The  three 
ears  were  grown  under  equal  conditions  and  gathered  on  the  same  day  to 
show  differences  in  maturity. 


b.  Two  inbred  strains  of  dent  corn,  No.  1-7-1-2  at  the  right  and  No. 
1-6-1-3  at  the  left  and  the  first  generation  cross  in  the  center  showing  the 
differences  in  maturity. 


PLATE  VIII. 


a.  Two  inbred  strains 
of  dent  corn,  Xo.  1-6-1-3 
at  the  right  and  No. 
1-7-1-2  at  the  left,  and 
their  first  generation 
cross. 


Mm  H^fe 


IfMw  ■     HIM  1  v . 


b.  An  inbred  flintfand 
an  inbred  dent  corn 
compared  with  the  first 
generation  cross. 


PLATE  IX. 


a.  Seeds  of  two  inbred  strains  of  corn  rand  the  seeds  produced  upon  the 
first  generation  hybrid  plant  in  the  center.  The  second  generation  plants 
grown  from  these  large  seeds  have  an  advantage  over  either  the  parents  or 
the  first  generation  hybrid. 


b.  Two  inbred  strains  and  their  first  and  second  generation  hybrids. 
From  right  to  left  they  are:  inbred  strain  No.  1-9-1-2,  No.  1-7-1-1,  (l-9xl-7> 
F2  and  Fi. 


PLATE  X. 


a.  The  same  two  inbred  strains  and  their  first  and  second  generation  hybrids 
as  in  IX  b.  From  right  to  left  thev  are:  inbred  strain  No.  1-9-1-2,  No.  1-7-1-1, 
(1-9  x  1-7)  F2  and  Fi. 


b.     Same  as  above — ten  plants  of  each. 


PLATE  XI. 


a.  Selfed,  reciprocally  crossed  and  out-crossed  seeds  obtained  by  pollinating 
plants  of  three  different  strains  with  a  mixture  of  yellow  and  white-carrying 
pollen  from  the  plants  which  bore  the  two  ears  shown  below,  showing  the 
ratio  and  distribution  of  the  two  different  kinds  of  seeds  produced  on  each 
ear. 

(The  seeds  resulting  from  the   "yellow"   pollen  were   colored  by  hand  on  all   three  ears.) 


b.     Seedlings  showing   the   rate   of  growth  and  the   amount  of  germination 
of  selfed  and  crossed  seeds  from  the  same  ears  from  five  different  plants. 


PLATE  XII. 


a.  The  first  generation  cross  of  an  inbred  strain  which  lacks  brace  roots 
but  has  the  habit  of  branching  freely  from  the  base  of  the  plant  (shown 
at  the  right)  with  an  inbred  strain  (shown  at  the  left)  which  has  well  de- 
veloped brace  roots  but  does  not  branch  at  the  base.  The  three  lots  of 
plants  have  resulted  from  three  seeds  each. 


«T9  ■-  q 


i>: 


% 


b.     A  closer  view  of  the  roots  of  the  plants  shown  in  the  above  illustration. 


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